Rarely. Epic Games Store has given GTA V for free twice (May 2020 and limited promotions). Other than that, no legal permanent free version exists.
No. Official digital releases are not distributed as standalone ISO files. The only legitimate ISOs are those you create yourself from your own retail discs. Gta 5 Iso File.7z
GTA V frequently drops to on Steam and Epic. For the price of two fast-food meals, you get: Rarely