The Gungan Army was the military force of the Gungan people responsible for protecting their cities and colonies.
In previous times they primarily engaged in skirmishes with Naboo's Human settlers in more recent times the two species, Humans and Gungans, have chosen to work together to defend their planet from external threats.
During the Trade Federation invasion of Naboo, Padmé Amidala convinced Boss Nass to join the Naboo to fight the Trade Federation army. The Gungan Army, led by Jar Jar Binks and General Roos Tarpals fought the droids forces valiantly but eventually found themselves overwhelmed, had it not been for the actions of a young Anakin Skywalker who shot down the control ship, the battle would have been lost.
Later, after the start of the Clone Wars Jedi Master Yoda sent a request to the newly elected Boss Lyonie to send aid to Mon Calamari for help after the Republic's clone forces were captured. Lyonie considered the request while the Republic commanders waited desperately for a reply knowing full well that the naturally aquatic Gungans were the last hope for victory in the important Mon Calamari campaign. Luckily for the Republic Jar Jar Binks was successful in persuading his people to answer the call by saying that if the Gungans were in similar circumstances Senator Admidala and the Republic would not have hesitated to send help. So Binks led the army and fought the Quarrens and Separatist droids. The battle lasted for a some time until the army was captured by the Separatists, with Prince Lee-Char still missing the battle appeared to be a victory for a CIS. Then Lee-Char emerged from hiding with newly Calamari allied Quarren warriors who vanquished the droids once and for all.