Other than occupying the same support race slot in the F1 calendar, F3000 has very little to do with GP2.
It's Bernie's series, it follows F1, and 20% of all GP2 drivers have raced in F1. Sure back in the day F3 was the proving ground, but GP2 was designed to be the stepping stone. And it worked well at the start. But your right, the testing ban ruined it.
1994: Jean-Cristophe Bouillion, David Coulthard, Massimiliano Papis, Pedro Diniz
1995: Vincenzo Sospiri, Ricardo Rosset, Tarso Marques
1996: none
1997: Ricardo Zonta
1998: Juan Pablo Montoya (loaned to Ganassi first)
1999: Nick Heidfeld
2000: Fernando Alonso
2001: Justin Wilson, Mark Webber, Tomas Enge
2002: Antonio Pizzonia
2003: Giorgio Pantano, Nicolas Kiesa
2004: Vitantonio Liuzzi, Robert Doornbos, Patrick Friesacher
I don't think moving up from F3000 was an issue, but usually the mega-stars were noticed before then.