It's a milled flat surface mating with another milled flat surface. The area is so small there will be no discernible distortion.
Silicon will seal it perfectly well.
However if you have measured for a stopper, then removed the old gasket before refitting, make sure that
1. no gasket bits drop into the engine, and
2. you factor the missing thickness of gasket into the stopper length calculation.
And you'll have saved the weight of the gasket!
Gasket thickness.
Re: Gasket thickness.
It's not falling off, it's an upgrade opportunity.