Most military geckons are bred to be large for both mounts and battle, usually being the size of a car or bus. The smaller versions are used for on field medics, spies and rooting out tunnels.
When not in battle, they are docile and can almost be a pet. They will stay like this until told a trigger word, using a specific word to the legion they are drafted in. Then they will tear through ranks mercilessly, a mindless killer. Luckily, there is usually a trigger word to stop them.
They're nowhere near as smart as wild geckons, and if any are they hide it well.