Garage gable fan proper installation
I hate adding anything to the roof, it just gives a place to leak. 

That truss you need to cut is non-structural. It’s made similar to a truss, so it will match the pitch, etc. Often carpenters will build a wall to match the pitch. 

Therefore, yes, you can cut into that wall, cut through the middle 2x4 web (flat stud) and install a vent. (Make sure you don’t cut the roof joists.) However, I’d add a 2x4 around the opening to stiffen the area where the vent will be installed. 

However, I’d verify that that side of your house is not the windward side. (I’d prefer the leeward side.) Wind driven rain could push through the vent causing multiple problems.