Junction boxes must remain accessible. You can put a blank cover plate across them, that will be fine.
Better to put the GFCI protection before the sink
I would go to the next receptacle upstream (toward the service panel) from this receptacle and look at the wiring there. If it's one /2 Romex cable coming in, and one /2 Romex cable coming out, slap a GFCI+receptacle there. Or, you can go all the way back to the panel and slap a GFCI+breaker there - however these tend to cost a fair bit more.
At that point, tripping this GFCI should shut power off not only at the sink receptacle, but also to the wires leading up to the sink receptacle. That way if water is splashed on the box, everything is protected. If that's true, I don't care if a receptacle is above a sink.
Use surface conduit to relocate the receptacle
If you don't have wall backside access, and want to use that box to bring power to, um, better receptacle locations, you can come off that box with a surface conduit adapter or starter plate. This sits proud of the wall, and gives you side ports where you can launch off with Legrand Wiremold, Rigid, IMC, or plain EMT (depending on the adapter). Then you can carry the wires on a (granted, unsightly) surface-mounted conduit to wherever you please.
In conduit, best to use individual wires of THHN (ideally stranded). You're allowed to use whole Romex (sheath must stay on), but that's ...painful.