Taken word for word from the GW Golden Demons rules page from the UK Site.
All models should be the work of a single hobbyist. All work on models entered into the Golden Demon - including but not limited to painting, modeling, converting, after-market sculpting, and basing ? must be performed entirely by the entrant. Neither models obtained from someone else in any state of completion nor collaborative efforts may be entered. Anyone found violating this rule will be immediately disqualified and forbidden from entering future Golden Demon painting competitions.
I guess you can interpret that how you wish.
While I know many modelers who have purchased my power cables from me and used them on Games day entries and won, I cannot confirm nor deny it.
Now I know for a fact, it has varied from competition to Competition. I know in the UK models with parts from kits by companies like Tamiya have won, while this summer in Chicago an Ork Nob Biker was disqualified for having model tank parts on it from a Japanese company.
Sorry of my original answer was a bit short, I was trying to keep it simple.
Golden demons can be a very \"Gray\" area for rules.