Hello,
IMC uses a variety of different MIBs, and it really depends on the index you are monitoring. Interface utilization-related indexes are usually based on some calculations related to IF-MIB http://cric.grenoble.cnrs.fr/Administrateurs/Outils/MIBS/?oid=1.3.6.1.2.1.2.2.1
These OIDs are primarily used for monitoring physical interface counters, but I think GRE should work as well, as long as the device supports it. Here is an example from my Lab with an MSR3024, and as you can see the GRE tunnel is 64K, which is not a common interface bandwidth.
However I'm able to monitor indexes like Bandwidth Usage in %:
You can check the OIDs by using Wireshark to capture traffic to the device when you deploy the monitoring indexes, or at every 5-minute (by default) interval.