To this date, there is no single conclusive answer to the question as to why the universal gas constant was denoted by R. The most prominent answer was probably given by William B. Jensen, a retired professor from the University of Cincinnati.
When Donald R. Paulson of the California State University asked the question, "Why is the universal gas constant in PV = nRT represented by the letter R?", Jensen took it upon himself to represent and analyze the historical events associated with the naming of this constant. The letter "R" was first introduced in such an equation by a French engineer, Benoit-Paul Emile Clapeyron, one of the first persons to combine Boyle’s law and Gay-Lussac’s law. At this point, there is no conclusive data that can accurately suggest why he denoted it by "R"; which leads to the speculation that it was either chosen arbitrarily, or it originated from the word "ratio", or its French equivalents "raison" or "rapport".
The conversion factor between the centigrade and absolute temperature scales that was used by Clapeyron was slightly inaccurate and was later corrected by German physicist Rudolf Clausius. Clausius used the accurate experimental data gathered by French chemist Henri Victor Regnault, which lead to the discovery of the accurate value of the constant. Due to this reason and the tendency of the International Union of Pure and Applied Chemistry (IUPAC) to name constants after famous scientists, Jensen argues that it would not be inappropriate to name the constant "R" as "Regnault constant" in honor of Regnault.