The Gek (not the ones from no man's sky) are a menace in combat, being invulnerable to small arms fire and plasma weaponary (they treat plasma like its a mild chili) this is due to the other species on their home planet of Gekkiza having evolved to break the laws of physics, for example crabs exist galaxy wide not because of some evolutionary advantage but because the crabs of Gekkiza evolved to be faster than the speed of light just so that they can reproduce without the risk of the roe being consumed by other species. One species evolved to spit plasma, hence why the Gek evolved to consume it, their skin is also resistant to it. The Gek evolved to be 20 feet tall to intimidate predators on their home planet (a bit like elephants) despite this Gek are usually a quite jolly and kind spirited species, unless declared war on, for example the UNE tried to liberate them due to a 'translation error' that translated the Gek for elected leader (G'aadx) to dictator (G'axvc), the UNE got humilated on the ground and the heads of the UNE not approving orbital bombardment due to not wanting to accidentally blow up civilians, combine this with the Gek artillary battallions uncanny ability to hit UNE artillary positons within one barrage from the UNE battery. At the climax of the war at the battle of class'isa the UNE's ground forces where nearly entirely wipped out with their anti tank missles and their tanks being the only parts of the army able to pierce the Gek's hide, even then only at specific areas of the hide. This was learnt earlier during operation 'I shot the Sheriff' where the UNE's top special forces tried to inflitrate the Gek command centre and assassinate general sh'aeri N'vlai, to which the general easily decimated the entire squad sent to kill him, a squad comprising of the UNE's top special forces operatives after luring them into a corridor to prevent their escape. Also to answer the question, why balls, to say it simply this was a miltary innovation to counter the Lek infantey whose skin was resistant to conventional firearms and thus ball launching firearms where created after it was found that balls where effective at killing Lek in combat. 

The Gek-UNE war was swift, brutal and humiliating for the UNE