maybe here
http://vortekproducts.com/ourstore/BSA%20breech-seals
but seeing that these could be for UK made BSA's and your could be a Gamo made gun and take a Gamo seal
the barrel style breech seals can normally just be rotated but i would look at Gamo for that seal
could be wrong
so Knibb's list the part as 34580 and that is a Gamo part number
can't find one but i would call Gamo for the seal
rotate and buy one from the UK if Gamo doesn't stock many rifle use this seal
i'm thinking this seal from Vortek would fit after doing more searching
http://www.airgunspares.com/store/product/3198/BSA-Generic-Breech-Seal-JKI-Part-No.-16-1050/
http://www.airgunspares.com/store/product/5433/BSA-Generic-Breech-Seal-GAMO-Part-No.-34580-16-1050/
Have made a lot of breech seals for BSA springers from aquarium tubing. Just measure the id of the breech cut and get the appropriate size. Purchased some seals for an old BSA Scorpion pistol and they appeared to be clear, square cut plastic tubing. Hummm. This inspiration was confirmed by some pro tuner friends.
Anyway, a small slice slightly taller than the cutout squishes up nicely and lasts as long as an o-ring. Doesn't cost much to try.
All for getting a new seal.....but will admit to some old-school make-do. So far as I can tell, the Super Sport used the same seal.
Have you tired getting the seal out...making a little do-nut shim (cut from thn plastic)...put the shim in...put the orginal breech seal in backwards (for a new sealing face)?
Basically, it either seals or it doesn't....if it does, then you're good until a "real" one makes it to you.