Hello,
Without being able to see the area, it would be hard to tell you what problem the Gerbil is having. I would like to try to treat the area. This may just be an inflammed scent gland, abscessed scent gland or as you have said a scent gland tumor. I would like to see you clean the area with some Beteadine scrub, then apply some Neosporin w/o pain relief. This should offer some relief to the Gerbil. If the area does not seem t5o get better with this treatment the next step would be to have a Vet who specializes in Pocket Pets to properly diagnose the area and give some more effective treatment. Joan
The information and suggestions given are basic first aid measures. In an Emergency, Please Consult Your Veterinarian For Further Help. Joan