As being surprised that Linux was not named as OS, I contacted Top Hat Studios via Discord:
~~
jmbtux — Today at 2:38 AM # 26.06.2024
Frogun Encore is just available to GOG - currently it is given as Windows only - will the Linux version come soon (as Frogun is available for Linux) or is this not planned - which would be a pitty.
~~
... and got the answer:
~~
AoiichiNiiSan — Today at 2:48 AM # 26.06.2024
It should be on Linux and it appears that GoG's storefront management made a mistake - we'll try to get this fixed
~~

So with bit of luck it soon should also list Linux!

I just thanked the Distributor ...

The Linux install script has just landed on GOG:
frogun_encore_0_914_74446.sh

# Off topic:
# # So I am just waiting for the Linux version of Toy Tanks to arrive ... also locCoop.

And the direct response from the Distributor of "Frogun Encore" via Discord:
"Glad that got worked out! <Emoji: Thumps Up!>
Sorry it took GoG so long."

It is nice that some people care !!!