Older releases (primarily III, Vice City and San Andreas) will run fine on the Mac because the onboard graphics are newer than the base game, so it’s not an issue.
However, that changed with IV, which genuinely does require a GPU to stand a chance. With GTA V, the GPU requirements are much higher. Unless you somehow find it runs okay on Iris integrated graphics with the graphical load it has, you have no choice.
I can barely get V running on an i7-3770 XPS 8500 with the OEM Radeon 7570 (OEM Dell 1GB, which is essentially an OEM 6570) with Win8.1/8GB RAM/1TB HDD/240GB boot SSD. The GPU doesn’t have enough VRAM due to how insufferably bad the card is, and the vBIOS calls for system RAM - 512MB of supporting system RAM to be exact. Not a lot but the GPU calling for system RAM tells me AMD designed the card around light usage but programmed a system RAM pull fallback option to make it seem like a better card then it is. I also need to cut the resolution down tp a much lower than normal level to not upset the GPU due to the 1GB on GPU/512MB (1,536MB) system RAM pull GTA V sees.
nVidia did this back in the days of Intel GMA as well with junk cards like the 9500GT (256/512MB VRAM, so it was literally just a way to get dual displays on a desktop with one video output), but nVidia quit making these things while AMD kept it alive for a lot longer.
You will need an eGPU or a system designed to handle the load for V - NO WAY around it.