I just followed this medium post, but long story short:

Check out the origin/master branch and make sure my local master, is equal to origin, then push it to my fork (I seldomly do git pull on master).

 git fetch origin
 git checkout master
 git reset --hard origin/master
 git push foursixnine

I needed to add the remote that belongs to a complete different repository:

 git remote add acarvajal gitlab@myhost:acarvajal/openqa-monitoring.git
 git fetch acarvajal
 git checkout -b merge-monitoring
 git merge acarvajal/master --allow-unrelated-histories

Tadam! my changes are there!

Since I want everything inside a specific directory:

 mkdir openqa/monitoring
 git mv ${allmyfiles} openqa/monitoring/

Since the unrelated repository had a README.md file, I need to make sure that both of them make it into the cut, the one from my original repo, and the other from the new unrelated repo:

Following command means: checkout README.md from acarvajal/master.

 git checkout acarvajal/master --  README.md
 git mv README.md openqa/monitoring/

Following command means: checkout README.md from origin/master.

 git checkout origin/master --  README.md

Commit changes, push to my fork and open pull request :)

 git commit
 git push foursixnine