>>212512271I hate to be that guy, but on the Galactic Battlegrounds game this is explained.
The Gungans regularly used guerrilla hit-and-run tactics, their original plan for resisting the invasion involved attacking Trade Fed outposts in small groups and retrating back to the swamps, over and over.
They never would have gathered their whole army for a retarded battle head on, which was suicide. They knew it was suicide. But Amidala needed a diversion to get the main TF army outside of Theed, so that's the only reason they did it, banking on Amidala being able to seize the throne before they got crushed on the field.
Meanwhile the TF just dismissed the Gungans as primitives and they were eager for a confrontation head-on, which benefited them.