At launch GTAV for PC supports the 26-letter Roman alphabet (upper and lower case) and the numbers 0-9 - if your username contains anything else there's a good chance you'll encounter errors and problems downloading.
Rockstar say they are currently working on a fix but in the meantime there's a workaround:
"If you don't want to wait for a fix, you need to create a new Administrator User Account for your Windows system that only includes letters A to Z, a to z or numbers 0-9 from the basic roman alphabet," they say.
"Please note that simply renaming your current User Account will not correct this issue. Once your new account is created (or if you already have an account that only uses these characters) we recommend you restart your PC and log in with that account before starting or continuing with the installation process."