Step 1 : Generate id_rsa public key by typing : ssh-keygen -t rsa -b 4096 -c "youremail@com" on your command prompt.
Step 2 : Now open public key file and copy paste to gitlab at the ssh key section.
Step 3 : After that go to source tree and navigate to Tools > Add SSH key
Step 4 : Then check the connections via "ssh -v firstname.lastname@example.org"
Step 5 : If success, you can now clone , pull, push or check out within your branches.
Step 6 : Otherwise, try git CLI to get it done manually if (Source Tree) problem occurs.