You download a 500MB file named GTA5_ISO.zip . Inside is a text file with a link to a password-protected RAR on a shady host. The password is only revealed after you install a "download manager" — which is actually adware or a keylogger.
: Because the standard GTA 5 installation is over 120GB , many unofficial sites offer "ultra-compressed" versions (sometimes as small as 2GB to 8GB) to make downloading easier. Kevin Games Gta 5 Iso File Zip
These files are often compressed down to roughly 300 MB to 1.6 GB . You download a 500MB file named GTA5_ISO