If you start the B12 and Cymbalta together, and start to feel better, you won't subjectively know which is doing the job.

But if you are so uncomfortable, and miserable not starting the Cymbalta is only prolonging that.

It will take a month or more on B12 to see if there is any difference.

But I certainly wouldn't add in other supplements with either of these two things, yet.

You are already on gabapentin, and know how much that is working, right? (or not)

Not everyone who comes here has a definite low B12 test result already in hand.
All I am suggesting is that the low B12 when corrected may help alot.

Cymbalta does not work for everyone...and it remains the only antidepressant with potential toxic liver reactions. So considerations about that need to be considered. (benefit vs. risk).

If your gabapentin is working for you, you don't need to change it at this time. But this drug also is not reliable, and new studies show only about 30% of patients find it works for them.