I'm not the best at troubleshooting, so forgive me for sounding like the other end of any popular company's tech support line, but have you verified that your monitor output is coming from the GPU and not the motherboard, i.e. running GTA V on Intel HD Graphics 630 instead of your 1080 Ti? I've seen it way too many times not to ask you, so sorry.
Otherwise, hit up Reddit's PCMR section and ask there. Those guys know what they're doing!