If you have multiple SSH keys and want to use a specific one for a particular Git repository, you can do so by configuring it on the SSH config file:

# ~/.ssh/config

Host gitlab.foo-repo
  HostName gitlab.com
  IdentitiesOnly yes
  IdentityFile ~/.ssh/gitlab_rsa.foo-repo

Here we have two key points:

1. The Host line is the name of the repository. It doesn’t need to be a real hostname, but it needs to be unique. The trick here is to use that domain as the git URL domain. Later the HostName line is the actual hostname of the repository, like git@gitlab.foo-repo:user/repo.git. This is the one that will be used to connect to the server.
2. SSH by defult will try to use all the keys available in the ~/.ssh folder, starting by the default key (~/.ssh/id_rsa) and then the rest of the keys. So, if you already have another key for the same server, it will try to connect with that key first. The trick here is to use the IdentitiesOnly yes line, so it will only use the key specified in the IdentityFile line.