Step 1: Generate a key on your computer with the following command:
Step 2: Copy the key from the new generated file. You can print it with: cat /c/Users/<your_name>/.ssh/id_rsa.pub. Make sure you print the .pub file!
Then copy the part from 'ssh-rsa ...' until your email (the last copied character is probably a '=').
Step 3: Go to Gitlab->settings->SSH Key (https://gitlab.com/-/profile/keys). Then add a new key and paste the copied key.