The IGP of the 5700G is based on the Vega generation of graphics, which launched in 2018. In addition to being older technology, it has only 8 compute units and 512 shaders, and is restricted to the bandwidth of your system memory.
The system requirements for the FiveM mod for GTA5 are the same as the original game. The minimum required graphics listed is an HD 4870. If you compare the performance of the Vega 8 to the HD 4870 you'll find they're very similar.
https://docs.fivem.net/docs/client-manual/system-requirements/
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT