When you create a project on Gitlab, you have to set ssh key on it.
Let's see how to set on the page.
1. Create public SSH Key
1) Download PuttyGen
2) Click Generate button and rolling mouse on the application
3) Copy the SSH Key on the red box (public key)
2. Register SSH Key on GitLab
Move to "Profile Settings" page
"Click SSH Keys" Link
Paste the SSH Key that you copied on the textbox that it has been created
※ PrivateKey has to be kept for push and pull command. In SourceTree, PrivateKey can be set as below.