Ah, don't forget, if you want your gitlab server to do the CI testing you need to install all the tools for it. I made that mistake at first too.
I installed php, phpunit, composer, phpcs, and phpmd globally.
Now, at minimum all you need is composer because the other tools you can add as dev dependencies on your project composer.json. In fact, laravel already pull down phpunit so you will have that one in:
So just do the global composer install (so it is in somewhere like /usr/local/bin/) and the CI runner should find it and be able to execute for you.