What kind of wiring/switch to use for gas fireplace switch?
A couple of alternatives to a 120V light switch:

A good quality sealed gold plated electronic toggle switch.  There's a lot of different kinds.
A smart relay switch.  EG ZWave or whatever you want.  About $40, which is more than a bit of wire and a cheap light switch, but much less than a new fireplace remote and you would then have remote control from your phone and would not need to install a wall box as it would all go under the fireplace instead of the existing remote.   Not a regular smart light switch, that would zap your fireplace with 120V but a &quot;relay&quot; or also called &quot;dry contact&quot; switch.

And ... by FAR the best thing about Option 2 is that combined with a taught Alexa skill you could simulate The Clapper.   Instant romance!