Problems with public key, from Azure Ubunto VM to Gitlab

I have a dual setup running between my own local machine (OSX) and an Ubunto VM running on Azure. I develop locally and push to a company Gitlab account using a ssh key that has my personal credentials. On the Ubunto VM, I have another company based ssh key with different credentials. Both keys have been added to Gitlab.

When I push/pull/clone from my local machine, it works great. No problems.

When I try to push/pull/clone from my Ubunto VM, I get an error:

Cloning into 'testproject'...
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

But, when I run ssh -T git@gitlab.com I get a welcome message, which, if I’m correct, indicates that the keys are setup correctly?

My ~/.ssh/config looks like this on my Ubunto VM:

Host gitlab.com
RSAAuthentication yes
IdentityFile /home/username/.ssh/id_rsa

And my .git/config looks like this:

[core]
	repositoryformatversion = 0
	filemode = true
	bare = false
	logallrefupdates = true
[remote "origin"]
	url = git@gitlab.com:username/testproject.git
	fetch = +refs/heads/*:refs/remotes/origin/*
[user]
	name = username
	email = username@company.com

Any input would be appreciated!