I've never dealt with GC but I would have a hard time asking any company to warranty a coilover sleeve for a seized set screw. I look at something like that as a maintenance item. I would think that a warranty would at most cover manufacturing/material flaws. These are "race" parts after all.
Not trying to be an a-hole but it's warranty requests like this that make it hard for folks when they have a legitimate (IMO) clame.
Between heat, penetrating oil and, at worst, drilling it shouldn't be that hard to fix the thing.
-Bob
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