Yes Garthe does call him brother. Because he felt his father basically chose Michael over him, letting him rot away in prison and giving someone else the legacy that was supposed to be his.
In the scene, you can hear the contempt in his voice as he says brother. Hasselhoff was pretty good at the good / evil difference between both characters. The only visual difference between them was the facial hair which as you know in Futurama Bender's evil twin has the facial hair (even though I think Bender turned out being the evil one). lol
I really liked what you guys did in the game though. I'm always down for more Knight Rider stuff, official or fan made. There's a YouTube channel called Lampoonery that uses kind of like deepfake and AI voice stuff giving a new life to the show combined with Fast and Furious, A-team. Theres another YT channel called Augur89 who recreated Kitt in GTA V with a crazy amount of features.
The cool thing about Garthe going over the cliff was it did leave it kind of uncertain if he was really gone. In the first episode with Kitt's evil twin named Karr. Karr ends up "blowing up" at the end of the episode. In the 2nd episode Kitt vs Karr you find out he actually just got buried in a bunch of sand after driving off a bridge. It was always my hope to see Garthe come back and instead of Goliath he would be driving Karr. That would be the ultimate face off of good an evil.