It's pretty much impossible to provide a precise number of frames per second you may be able to gain with any given hardware change. If going by benchmark comparisons*, it looks like the 1060 has roughly double the performance of the 1050. This would make sense since it has double the CUDA cores (1280 vs 640). The 1500X, versus the 8120, appears to come in at around +60% compute performance (with an aggregate sample of Fire Strike scores showing a +113% physics score improvement). Taken with a grain of salt, I wouldn't be surprised to see about double the FPS. This is all conjecture since I don't have a 1050 and a 1060 to bench against each other in GTA, nor those CPUs. Obviously, bumping up the graphics settings will diminish the gains.
* never consider benchmarks to be a definitive and foolproof way of determining system performance increases