The search for " highly compressed 20GB" reveals a mix of experimental modding projects and common internet "repacks" that claim to shrink the massive game for low-end hardware.
Files claiming to be GTA 5 under 30GB are almost always traps. Here is what typically happens if you attempt to download them:
Since the installer needs to bypass Windows Defender (by modifying registry keys and injecting code), malicious actors can easily hide a backdoor. Once installed, they can lock your files or steal saved passwords from your browser.
Go to \Documents\Rockstar Games\GTA V and open settings.xml with Notepad. You can manually drop shadow qualities and reflection qualities below what the in-game menu allows to free up your CPU and GPU pipelines.
Grand Theft Auto V PC system requirements - Rockstar Support