The gaskets are supposed to be a stretch fit. This means that the cross bars that go across the rads surface with nothing beneath them are tight enough to pull upwards slightly and press against the fan to ensure an airtight seal between each fan to prevent airloss in case there is positive pressure or a cause of back pressure applied to the rad by the chassis.

The gaskets only ever need stretching width wise - simply stick one edge down, aligning the inside of the thin part of the gasket to the inner edge of the sidewall where it folds round, then stretch over and stick down the other side. That ensures it doesn't snag the blades of the fan.