Currently, it is impossible to specify a material for a given portals' button.
This has been brought up as a potential feature both upstream (PseudoKnight#7) and on our discord.

There are two ways this could be implemented:
1 Adding a customization option to .gate designs as was done upstream (2628553)
2 Allowing players to supply their own buttons when building the gate, as is done with signs.

• Note that, as buttons are supplied automatically, code modification would be needed to prevent the button from being replaced with STONE_BUTTON

I personally prefer option number 2 as it is more customizable for players, less overhead for admins, and slightly more intuitive.