Gaud's attack plan was very simple and effective, just the style he loved.
The same way that Rex, Aragorn, and the others researched his personality through Max and Stark to better implement plans to counter every move that he made, so also did he research extensively about his enemies.
Rex's time in the Gundam Shelter was short which meant that not a lot was known about him yet, but Gaud was able to work with the little that he got.
From the little that he was able to get about the 3 gang leaders of the Berserk Gang, he knew that they were not fools that he could easily outwit.
It was definitely not going to be easy, but he still had confidence to outwit them. It would just require more effort and planning on his part.
After knowing about them, he knew instantly that they'll research him and plan contingencies to counter his plans.