I wanted to display the FPS counter for this game but it doesn't seem to support the GOG overlay menu (Shift+Tab).
Is it just me?

Before you ask, yes, the Overlay is enabled globally (e.g. it works fine in Disco Elysium).
And no, it isn't disabled specifically for this game.

I tried launching the game with the default Desktop shortcut :

> "C:\Program Files (x86)\GOG Galaxy\GalaxyClient.exe" /command=runGame /gameId=1417618902 /path="C:\Program Files (x86)\GOG Galaxy\Games\Firmament"

and from the GOG Galaxy "►Play" button, but same result.
I tried dx12 and dx11 modes, same result.

Is it just not supported at all?

I disable the overlay on purpose as those can cause issues. Probably the only reply you will get and its that lol :)

There's always msi afterburner or whatever.

Edit - I did test this as I bought more and faster RAM and wanted to see if increasing the RAM mhz would make any FPS difference at all. The overlay isn't working for me either after enabling it. Afterburner does work great although just annoying to setup.

Side point: RAM mhz can be entirely worthless. I think it just depends but I know people originally suggested for my CPU that it benefited greatly and it actually makes zero difference for me. I confirmed its accepted by the board and running at chosen mhz as well. I'm now running in mixed mode at full stability and same fps. To keep it simple. I could expand on this but won't and it would get too long.