first you have to create an SSH key ... the details are as follows:
in Terminal do $ ls ~/.ssh: if you have a file named id_rsa.pub or id_dsa.pub you already have a key
otherwise:
ssh-keygen -t rsa -C "Comment for key"
name: press Enter
passphrase (i.e. password)
then go to Gitlab site ... personal icon top right
on the local machine, do this : $ cat ~/.ssh/id_rsa.pub and copy the entire contents
in the box under Add an SSH key / Key, paste the contents.
give a simple explanatory title in "Title". Set "Expiration date" to 31/12/2199
click "Add key"
on the local machine do this, probably in the directory where your .git file or directory is located:
$ git remote set-url origin ssh: