If you have deleted the papers from your profile only, then you can restore them by just finding the paper as it will be available in the profile of your co-authors. However, if you have deleted the paper completely from research gate, then you need it upload them again.
I posted the option stated by Varun and then deleted. There is option called 'Claim authorship' in your coauthor's profile. I used that earlier but I could not locate this time.