This petition was rejected

The Government e-Petitions Team gave the following reason:

The decision about whether to host adverts for free animals is a matter for Gumtree, which is a private company.

You could try contacting Gumtree directly to ask them to change their policy.

Alternatively, you could start a petition asking Parliament to pass a law restricting the sale or giving away of animals on the internet. You'll need to give your full name to start a petition.