As Jorats said, most people on here believe in making a commitment to your pets for their lives. Circumstances need to be very extreme for most to ever consider rehoming a pet and many on here rescue animals. I can't add to what the others have said so I'll give you some info.
Many people with allergies find that they are allergic to the type of bedding being used.
Just so you know, if you are allergic to a pet that does not mean that you need to rehome it.
There are quite a few people on here that are allergic to their rats but do things to deal with their allergies and would never consider giving them away.
For airborne allergies, many people use an air filter with a permanent hepta filter and replaceable carbon filters (Not an ionizer). If there is a problem with contact allergies from their little claws, wear long sleeves, and there is ointment you can use to help your skin heal.
Gerbils will do fine in a large clean cage with friends, toys, good food, water, vet care etc. Also, they really should not be in the same room as rats and the rats should never have access to them or to the gerbil cage because the rats will kill them .... horribly. To my knowledge, gerbils do not require the same level of interaction that rats require.
It is difficult to find good homes for small animals but if you really want to rehome them, I hope you only let them go to a wonderful home where people will make the commitment to care for them for the remainder of their lives. Make sure you use a contract with a return clause saying they must be returned to you if the person no longer wants them - this will help to protect them. Many animals rehomed on kijiji are rehomed many times and end up in horrible situations.