Aggression depends on a lot of factors. Generally the cichlids in the tank will have already decided that it is "their" tank, and newcomers will be chased out. Only in an aquarium, there is nowhere to go, so they end up getting harassed. Oscars are not super high on the aggression scale normally, so you will probably not have as hard of a time as someone who had say, red devils.
The best thing to do is probably to do a water change before you add the fish, and comepletely change the décor around. Hopefully this will trick your fish into thinking it is no longer all their territory. Also keep the lights off for the first day so the new guy can hide. Adding one cichlid at a time can be tricky, but GT's are usually a little meaner than Oscars, so you should be alright.
