It's generally a bad idea to try and close the gap by just squeezing the parts together. It introduces stresses in the flange and can cause fatigue cracking down the road. Putting sealant in does nothing to fix this issue.
I like David's idea to try bending the flange back first. You won't get it back to its original position, but you might get it closer. Then you get a piece of aluminum of an appropriate thickness to use as a shim. Try to get no more than 0.005 in. of gap before you shoot the rivet. Adjust the length of the rivet as necessary to account for the additional stack up thickness.
If the shim is bare aluminum you should treat it with Alodine. If it's clad you can leave it alone. Alternatively, sealant around both sides of the shim will prevent moisture ingress and subsequent corrosion. Either way works.
RV-14A Kit#140433, N393AJ Reserved
Completed: Vertical Stab/Horizontal Stab
Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
Dues paid on 10 October 2018
Member of EAA Chapter 92
Pet peeve: "Lose" is the opposite of "find". "Loose" means "not tight".