Quake is C with some assembly, which can't be used in C#.
Code conversion from one language into another happens very often.
Besides, the ASM is only the sped-up 256 color software-renderering code so that's really irrelevant.
GoldSource has a C engine and largely C++ game logic. Quake code would have to be heavily modified to even be usable, to the point where it has to be redesigned from scratch to be useful.
I beg to differ. The game-logic from Deathmatch Classic, although it may be C++, is actually converted from the Quake 1.06 progs code.
A language that doesn't even have a concept of integers.
If Valve didn't own the licensed Quake code, that'd be, for example, a breach - if it was based off the GPL-released code and they didn't adjust their license properly to comply.
and the Quake license states this:
What you quoted is from the paragraph about
modification of GPL2 code.
This does not allow you to take Quake code and put it into non-GPL works under any circumstances. It applies to your
own modifications and when you can relicense them as something else. The license clearly states that once your code depends on working with GPL code, your code has to, I quote:
"be on the terms of this License"
Thus, no licensing issues.
As long as you don't use any Quake code (converted, reformatted, copy&pasted etc.) you'll be fine.