It's up to state legislatures to certify the results, they could conceivably just pick a bunch of faithless electors that vote for Trump but that's unlikely. Also, congress and the senate can accept or not accept each states electors - meaning the senate could just say "we aren't counting Pennsylvania and Arizona" or whatever combination in a last-ditch effort to get in Trump (also, there's no real mechanism for how to deal with the Senate and Congress not agreeing in a joint session and no procedures or precedence to guide them).

It's very unlikely that the GOP will do any of the above for Trump. But it is cool to know that there are a number of levers they get to just veto an election in favor of themselves.