My brother is playing this and losing a lot. Git Gud isn’t working for him, and to shut him up I wanted to let him use cheat codes. Unfortunately pressing F8, typing code in allcaps and pressing enter doesn’t do anything. Can anyone help?

I think it's because this one is not GunMetal: War Transformed by Mad Genius Software, but Gun Metal by Rage Software that was also released on the OG Xbox in 2002/2003.

While it may feel similar, I think the homophony is accidental.

Apparently there were XBox cheats that involved clocking the analog sticks, but I haven't tried them on PC yet.

Here is the GameFAQs page if you want to try them.

https://gamefaqs.gamespot.com/xbox/516236-gun-metal/cheats

If they work that could always be neat, but apart from the joke briefings, I don't think I'll have much use...
Well actually mission skip can help on some of the annoying missions where you're besieged and have to avoid reaching a certain threshold of base destruction.