Yeah, flatpak - a real PITA
It probably depends on if you already have a regular existing Gimp 2.10 installation. This in a Mint 20 VM and a Gimp 2.10.36 flatpak. (A bit old but I can not be bothered to update, just useful for demos)
Two possibilities.
(1) The flatpak gimp_gmic_qt plugin.
Attachment:
01-flatpak.jpg [ 135.41 KiB | Viewed 405 times ]
A bit strange, the new gradient appears in the old Gimp 2.10 gradients folder (2) not the .var (1) one which you might expect.
(2) RobA's script. This did not show when in either of the regular script folders .config... or .var.... I had to make a scripts folder in home (3) and add the location to the Preferences -> Folders ->
Gradients scripts paths.
Attachment:
02-flatpak.jpg [ 140.49 KiB | Viewed 405 times ]
It does work, beware of the dialog "popping under" the Gimp window. Refresh the gradients for a write to... the old Gimp 2.10 gradients folder again. (4)