for me resuming download works, maybe u can only lose a part of files when a new update from rockstar comes out
cool !!
because when i was downloading it the first time and cancel/resume
i get this error during downloading "connection to download server lost... Reconnecting"
and fixed this with deleting the ".part" files from the game directory every time before i resume... and it was resuming with a problem.
but when i finished i faced a "freezing error druing launching the game"
when i contacted Rockstar's support they said that i faced the error due to the existance of some courrpted files during download... maybe deleting the ".part" files was causing this ..
sorry for wasting your time

but to sum up., when i try to resume the download should i resume without deleting the ".part" files ?