Where can I find the standard definition for GPG ECDSA signature? I am playing around with GPG which I'd like to use to sign an integer and use the resulting signature binary in another piece of code.
So far I have created a simple PGP packet parser with which I think I can successfully extract the elliptic curve point representing the public key.

Comes then signature parsing from the PGP message and which I haven't been able to interpret so far, finding no hints in RFC4880 or 5480 which define all bytes up to the last 68, the signature representation. 
But how should this 68 bytes be parsed exactly? Which RFC would be the one to look at?
An ECDSA signature in PGP is indeed encoded as MPI(r) MPI(s)
This is confirmed by RFC6637 which refers back to RFC4880. This is made more clear in draft RFC4880bis which incorporates the former. 

Multiprecision integers (MPI) format is itself described in RFC4880, section 3.2