Learn now to navigate the Linux Terminal
You can open a linux terminal my clicking on the Terminal icon at the bottom of the graphical interface or use the Applications menu.
All you need to do is to learn how to go one level up or down and go to the right directory.
To go one level up type
cd .. (make sure there is a space between cd and ..)
To go down type (e.g.) cd OA_Gladstone_mr/
cd /OSM/MEL/OA_ALANTIS/input/AltantisModelFiles/GAB/
./runGAB.sh
There are heaps of really good tutorials on the web about using a terminal.
https://www.howtogeek.com/140679/beginner-geek-how-to-start-using-the-linux-terminal/