I have used both buprenorphine and gabapentin and transdermal fentanyl for my cat, but it was for oral cancer not pancreatitis. Still, severe pain with the cancer so I'll add some info my cats experienced about side effects and perceived pain management. Also used bupe (buprenorphine) with a cat with severe stomatitis that due to advanced heart disease could not have dental work to remove her teeth.
The buprenorphine was given orally by syringe, into the check pouches, 3x day. A glass bottle of bupe is better than prepared syringes for storage up to 180 days. Bupe degrades too much if stored in a plastic syringe per this article.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5517336/ I stored the bottle of bupe in the fridge and it came in dark glass from the pharmacy. No noticeable side effects in either of my 2 cats. Good pain management.
Gabapentin is very bitter, so you need to get it in a capsule form or a small enough pill that does not need to be cut or easy to hide in a pill pocket. Often difficult to get gabapentin in a small enough dose for a cat, and it's often compounded into a liquid for use. Foaming and mouth drooling are likely if you give the liquid. Chase with a syringe of liquidy food if you can. Expect grogginess, for your cat to sleep more and be harder to wake up as side effects. Cat will often be unsteady on their feet. Something to consider if your cat likes to jump up onto a cat tree or counters or a table, is that the wobbliness may make them miss their target and fall and injure themselves.
I was using both the bupe and the gabapentin towards the end of life to provide additional pain management.
Have also used a fentanyl patch on a cat. That is NOT something you can have hanging around at home and use when needed. The patch had to be applied by the vet/vet tech, well wrapped, was only good for 3-4 days and then had to be removed and a new patch applied at the vet. Fentanyl is highly regulated and difficult to get, even at the vet, not meant for a non-vet type person to apply or remove as the transdermal patch needed careful handling with gloves. Did not see significant difference in pain management between the fentanyl and the bupe for my cat so I chose to use the bupe which could be administered at home.