Can't help you with the brand of the instrument, because mine was changed by the previos owner to a turotest model. All the instruments are pretty standard with a 52 mm breaktrough diameter. For the Volvo ones (which i think are branded VDO makes) you can get replacement front rings (i think including the seal), but this needs exact determination of type to get the right one. But you can also cut the sealing ring from a 1 mm think neoprene foam piece, using a sharp cutter knife or a pair of scissors.
I had a similar problem with an arrangement like on your photo, where i suspected an instrument to leak, but it showed it was the seal of the compass (seems to be the same situation as on my boat) .
Worth to check before ripping the fuel gauge out. Good luck!