Where can I find the standard definition for GPG ECDSA signature?
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