Well, Derby is quite the manipulator himself, maybe he thought he could use Gary without allowing him to do the same? Or maybe Gary just offered to 'help' with that fetish he has...
The two never really actively did anything and there was never anything connecting Gary to telling Derby to make the mark or any sort of plan between the two of them.
I see two possibilities:
1. Gary witnessed Jimmy tagging city hall and used it to his advantage or the version I prefer...
2. Considering the theme in chapter 5 of 'Gary says something, so it's automatically true' he just told Crabblesnitch Jimmy did it without actually ever seeing him do it.
If Derby and Gary did plan that together to set Jimmy up for the fall we will never know.
Thing is I don't think Derby ever showed a particular interest in school politics aside from the traditional Greasers grudge. For example, the Jocks supposedly rule the school and yet he won't make an alliance with them. Frankly Derby does not like to lose at anything he cares about (Harrington's never lose).
Conclusion: Derby doesn't give a crap about the hierachy or the politics and probably didn't give a crap about Gary, no matter what he did.
