Hey guys, I'm wondering how can I sync to the server-side a block that was removed on the client-side using a GUI?

You need for this packets:

 

http://www.minecraftforge.net/wiki/Packet_Handling

 

It's a bit hard too understand, so just ask again if you cant get any further with you code.

You don't even have to mess with packets themseles. Diesieben has an incredibly simple to understand tutorial on how to use the SimpleNetworkWrapper.

To be a bit more detailed, you need to use packets (as mentioned above) so you should implement the simple network wrapper according to diesieben07's tutorial linked above. 

 

After that the idea is that in you GUI you don't actually delete the block but instead it sends a custom packet (maybe call it MessageDeleteBlock) to the server and in that packet you would send the coordinates of the block you want to delete.  Then in the message handler on the server side, when it receives that packet type, you would set an air block at the coordinates you get from the packet.

I think I got it working, although I have a question.

 

It just works if I don't return anything in the onMessage() method. If I return the "massage", it says that an integer is (obviously) bigger than a byte, and the game crashes. Should I leave it null?

Whatever you return in onMessage should be the reply if you need one. Otherwise return null.