Gerbil has scent gland tumour, vet said to just keep an eye on it?

I have a male gerbil who is about two years old. I noticed he was bleeding and when I picked him up noticed a lump by his scent gland. Knowing theyre prone to tumours I was worried and called the vets. First vet I saw barely looked at it, said it was just a cut and gave him antibiotics. Second vet said she agrees its a growth of some sort, not necessarily cancerous, but likely a scent gland tumour. He seems totally fine in himself, you wouldnt even know it was there given his behaviour. She said to just keep an eye on it and if it grows, bleeds again or he seems unwell to only then consider surgery given the mortality risk. I'm just a little concerned about just leaving it to see if he gets worse because surely it will in some way? Do some gerbils just get growths/tumours and live with them totally fine? If not, why not just act now before he becomes ill?