W/out dismantling your boat, I wouldn't know for sure, but I think that keel strip is not supposed to be sealed to the hull. Although the gap from beaching isn't helping it isn't the major problem.
Along that strip that you think now has a larger gap, are there any 1/8" holes drilled in it. Right at the peak of the angle? Look all along the bottom if it, esp from midway & back towards the transom. Starcraft puts weep holes in that keel strip, to let any water that manages to find its way in, can get OUT to prevent the trapped water from freezing & rupturing it or damaging the hull.
W/out access to the inside of the hull you can't re-buck any loose rivets. But you could drill them out, attempt to reduce the small gap you think got opened when you beached it, and re-rivet the keel strip w closed end blind rivets. IMO, applying Gluvit to the gap won't solve the problem. And may cause other problems of trapping water against the aluminum hull. Freezing is only 1 of them. Galvonic corrosion is too.
Find weeping rivets & seams. Check the rivets & any near a weeping seam for loose, damaged or missing rivets. Rebuck or replace suspect rivets. Then re check for leaks. Any seams that still leak might be sealed w 5200/flight FROM THE INSIDE. Once the gluvit is dry, check again for leaks. Applied from the inside, any sealant will be covered & likely not seen. Both can be a mess to work with so on the outside it might show. Gluvit is epoxy based, so it must be covered & protected from UV.
Again, I don't think that particular gap is the problem, but nearby rivets might be. On my phone so I don't know if you mentioned what aluminum boat this is...
