Thanks for answering my question and checking in, Russ.
The vet thinks Gus Gus has a fibrous tumor. Gus Gus also has heart arrhythmia. They're giving him some medication to even out his heart rate and he is going to have surgery on Saturday to remove the tumor. The vet thinks the tumor will be pretty easy to remove.
I'm not sure about putting him through surgery because he's pretty old (2 years and 8 months). But I've had rats live to be 3 or 4 and if the tumor is in a good spot, it seems worth it.