The farmlands of 19th century Gelli were owned by absentee landlords, like Crawshay Bailey and the Earl of Dunraven, who would gain from selling the areas when coal exploration began. The first pit sunk in Gelli was in the 1870s, undertaken by the firm owned by Edmund Thomas and George Griffiths. The Gelli Colliery was then purchased by brothers, John and Richard Cory who deepened the pit further. The colliery suffered a mining accident when a gas explosion in 1893 took the lives of five miners. Subsequently the pit was sold to the Powell Duffryn Company, who owned the mine until the nationalisation of the mining industry in 1947. A second pit was opened by David Davies in 1877, the Eastern Colliery, though this closed in 1937.
As with all villages in the Rhondda Valley, since the end of the coal mining industry, the area has suffered an economic depression with a history of unemployment higher than the national average. With little employment in the area apart from retail services many people commute to work outside the valley.
Gelli is neighboured by the towns of Ton Pentre and Ystrad and due to the fact that the B4223 link road, that services Llwynypia to Cwmparc, goes through Gelli it experiences fairly heavy levels of traffic. It has also suffered in the past from flooding, due to its low level in comparison to the River Rhondda, and after two serious floods in 1960 and 1970, the council responded by improving flood defenses to the Rhondda Wall.
Gelli is also home to a landfill site which has been at the centre of controversial reports connecting it with birth defects in children born in the nearby vicinity. The reports resulted in a full scale environmental report, and the decision by the local authorities to cease the dumping of domestic waste at the site.