Okay I seem to be getting somewhere. I think the problem might lie in what version of Gallery 2 is being used. Note that the plugin calls for version Gallery 2.1 RC2a or above which can be found on the Gallery download site here: http://prdownloads.sourceforge.net/gall ... rror=jaist
I think the problem, Dalif, is that both of us were probably using 2.1.2 (no RC2a on the end).
Here is what I did:
I created a new subdomain called test and installed a fresh s9y and Gallery 2, but with php5 installed. Gallery 2 works just fine with this, but the specific version of Gallery doesn't like the G2embed plugin. However when I pointed my gallery install at my main Gallery that I use for my site - voila, it works. So I think the version of php may also factor in.
If you have time to try this out, let me know how it works out for you, Dalif. I hope that this may point you in the right direction.