If you're using OpenSSH, the IdentityFile configuration directive selects the SSH key to use.

Add something like this to your SSH config file (~/.ssh/config):

Host github.com
  IdentityFile ~/.ssh/github_rsa

Host gitlab.com
  IdentityFile ~/.ssh/gitlab_rsa

This will use the github_rsa key for repositories hosted at github.com, and the gitlab_rsa key for repositories hosted at gitlab.com. Adjust as needed for your key names and hosts, obviously.