Signature mismatch for secp256k1

[Louis Holbrook]

I'm trying to use gpg-agent to sign for cryptocurrency purposes, using
the secp256k1 curve. I've tried a few hashes, but one of them gets a
different resulting signature than other implementations. I've compared
against libsecp256k1 and nodejs elliptic library.

I won't post the code here, but I also put up a question o stackexchange
on the same topic which lists my attempt:

https://unix.stackexchange.com/questions/589730/gnupg-secp256k1-signature-does-not-match-other-implementations

Any pointers would be appreciated. I'm guessing it's some silly
oversight of mine, but I can't spot it.

Thanks.