GTP requires at least an IMSI and an APN to set up a connection. The IMSI is required to identify the user, and the APN is required to identify the network the user is connecting to.
There is a 1:1 relationship between IMSI and subscriber ID. It is possible to provision only one of these and the other will be accepted as the same value. If both are provisioned, they must be equal. Therefore, it is not possible to set up more than one GTP tunnel per subscriber.
APN can be provisioned explicitly per subscriber, or a default APN can be provisioned per VRF. If this APN does not contain an Operator Identifier (OI), it will be added automatically based on the IMSI.