Grammatical structure, pronunciation and many terms reflect a certain nearness to Corsican (many similarities with the southern Corsican dialects of Sartene and Porto-Vecchio) with which it shared some mutual influences due to close relationships in 17th century, but many words come from the Sardo logudorese variety of Sardinian, which was spoken in this area in the Middle Ages.
It is deeply controversial, and in fact discussed, whether Gallurese should be included in the Corsican language, as a minor form or a dialect of this one, or instead included (as now is) in Sardinian. An evident similarity, indeed, exists between Corsican (especially southern dialects) and Gallurese, and as evident as the distance from the bordering Sardo logudorese. Undoubtedly the two idioms are in some elements similar and quite certainly they are reciprocally influenced.
Sardinian language is deeply conservative, ancient, Latin-based and historically influenced from Spanish and Catalan. Southern Corsican should be derived by the idiom spoken all over the island before Italian and French hard influences (dialect of Bastia presents notable similarities to Tuscan). Before French domination, standard Italian was for centuries the cultural language of the island. Moreover, there was a progressive mutation of the languages between Sardinia, Corsica and Tuscany.
Academically, and in scholastical classification, Gallurese is often considered a Sardinian language for two main reasons (that could be controversial):
This genetical reason links the Corsican languages and the Sardinian languages in a remote Romania africana : only the true Sardinian dialects are the heirs of this former unity.
But in the recent regional law (No. 26, 1997) voted by the Sardinian Parliament, the Gallurese (with the Sassarese) is not considered as a Sardinian language but in the same category as Catalan language of Alghero or Tabarchino.
The occasions of contact between the idioms could be interesting, in order to know more about the genesis and the development of Gallurese. Before the Phoenician languages, the presumed Paleosardinian language of the Nuragici people should have been evenly distributed over the island, but archaeological evidence of the language is only found at a few points on the island.
Phoenicians came and invaded the island, only two main areas remaining free from their control and their influence, the two traditionally independent areas of Sardinia: Gallura and Barbagia. Here the contact with surrounding areas was broken, supposedly. So there should locally remain the influences of Balari (a Sardinian people that came from Spain), probably with protoiberic influences. Then Rome came, and defeated Balari, therefore Rome was in Gallura too, as it was in Barbagia. Maybe it could be presumed that when the Latin language entered, only some part of the idiom was different, so the difference could have came later.
The greatest difference could perhaps be born in the age of Giudicati, it could be Pisa, and the quality of the presence of Pisa to start separating the local speaking. Since the defeat of Arab pirate Musetto, 1000-1050, Pisa was in the northern Giudicato with the sole "disturb" of Genoa (and the Doria family), then it would have remained there as the main dominator. As known, we know most of medieval sardinian history thanks to the Pisan liber fondachi, the registry of paid taxes, which is so detailed to let us consider Pisan presence as a heavy influence.
From this point perhaps the languages begin to part, along time producing what lets now underline that there is some Italian language and some Tuscan in Gallurese. But talking about Corsica, the mere institution of a common Regnum Sardiniae et Corsicae (1297), more a formal political act than a concrete creation of power at the beginning, does not allow to consider that particular relationships were established between the islands just as an effect of this institution, being the main action of Aragon, at that time, the first conquer of Giudicati, mainly Arborea, and it took quite a century or something more. Pisans remained in Sardinia for a while, as their Romanesque architecture and some of their idioms attest.
It could be interesting to investigate if the Tuscan ingredients of Gallurese effectively came from this domination, because in this case, a Tuscan influence would have presumedly affected a Sardinian version, resulting in a modified Sardinian version (then it still could be a Sardinian version). It would consequently be classified as a Sardinian language for a genetical reason.
In this hypothesis we were considering Sardinian as if Corsican could modify it, but the contrary could be possible, as well, and there could also be a possible Sardinian influence on Corsican. It could also be that Corsican had been influenced by Sardinian. Or it could eventually be that they both were influenced by the same factors, received in their own territories separately and with different actions, finally producing not so distant results. External influences could consent this hypothesis: Sardinia had a wider population, and in its history has been more subject to foreign influences than Corsica. Invasions and taxes would probably have caused an eventual movement in the direction of the minor island, less probably the other way, even if the establishment of a few groups of Corsicans in Sardinia is known. Also, the little distance between Corsica and Tyrrhenic (tuscan) islands would let suppose more frequent practical contacts, while in Sardinia these contacts were more decisive on a point of local administration.
Contacts between the two islands were again intense in 17th century, when France entered in commercial relationship with Sardinia, provoking (not completely unintentionally) a certain contraband in the Bocche di Bonifacio (the narrow channel that divides the two islands); the need of using a common jargon on the coast is of evident relevance. Its diffusion however was not so wide, presumedly.
Similarities do exist also with Maremma, in southern Tuscany-northern Lazio, with which there were no relationships. All this would tend to suggest that a common evolution had interested the areas. Some authors like Maxia believe that, in analogy among themselves, all the coastal areas of Tyrrhenian Sea should have lived an evolutive moment perhaps at the same time. But the point is not obviously the similarity, because it would not be sufficient to a classification. Current central logudorese Sardinian is not similar, in phonetics, to medieval logudorese Sardinian, although no one would ever deny that it is the same language. How much did it change, from what was it changing, what happened in Gallura and from what Gallurese started, are still unanswered questions, by now.
A proposed proof of a common genetical root between Gallurese and Sardinian regards the turning to Italian language of both logudorese and gallurese people when they abandon their respective natural idioms: considering the relevant similarity in the use of their second language (Italian), it could be not immediately unfair to consider that the genetical root should be common, even if the "musical" result is different.