If you want a g2 to run as an external, you will need to do the recirc mod to get a good air pull, use a separate feed pump to push water to the skimmer, and do the gate valve mod to be able to feed the water back into your sump. all are relatively easy projects, especially for a skimmer you got for $1.
you can find all of these with a little search-fu on RC or just check out the asm skimmer club thread. tons of good info.
good luck!