I always thought a GBC power adaptor was 300mA, which would struggle for the gp2x which can soak upto 2000mA (2A) depending on application, overclocking, and tv out.
That is how much the GBC needs, but all the adapters for it I have ever seen far exceed that. I mean, come one, a 3v adapter that can only supply 300ma is pretty bad. It has always worked for me, just cut it open, swap polarity, and there you go! Note: Results may vary, if it does not work for some reason, dont blame me for making you ruin your gbc adapter.