How can I determine the GPG/PGP key ID from the RSA modulus?
It depends on the version of the GPG format. In V3 the Key ID it is indeed the bottom 64 bits of the modulus, but in V4 it's the bottom 64 bits of the key fingerprint, which in turn is the  "SHA-1 hash of the octet 0x99, followed by the two-octet packet length, followed by the entire Public-Key packet starting with the version field" according to the RFC.