In 100 BBY, the Grand Army fought off unknown invaders of Naboo. As a result, the Gungan tribes are united even closer than they were after the War of Gungan Tribes thousands of years before.
In 32 BBY, the planet of Naboo was invaded by the Trade Federation. The Grand Army was composed of thousands of Gungans and was commanded by Tobler Ceel under Boss Rugor Nass and the Bombad General Jar Jar Binks. The battle weakened the Grand Army but they succeeded nonetheless by preventing the Trade Federation's army from reaching Theed.
The Gungan Grand Army didn't use any vehicles or technology from other planets. Instead they took indigenous creatures such as the kaadu, domesticated them and then trained them to stay calm even in the frontline of battle. They then placed mechanical technology such as shield generators onto their larger beasts such as the fambaa.
This was economical to the Gungans as this saved the trouble of looking for reliable weapons offworld and the possibility of being corrupted by foreigners. The Gungans, although they did not use very sophisticated technology, did use a shield generator to stop all bullets and missiles. Droids, however, were able to physically walk through the shield.
The Gungans also used Bongos for transporting supplies.
The New Essential Guide to Alien Species states that the first Grand Army was formed in 100 BBY to fight off unknown invaders and unite the Gungan tribes. However, this conflicts with the fact that it was in existence thousands of years before during the War of Gungan Tribes and the Gungan-Naboo War.
Another contradiction in the Essential Guides is the existence of the Gungan-Naboo War. One source states that a war occurred between 150 BBY and 121 BBY. The New Essential Guide to Alien Species states that there was never a war between the Gungans and Naboo.