The most powerful assault rifle in the game; after ammo modifiers it's significantly stronger than the next best choice, the H&K P90c. On the other hand, the even stronger Vindicator minigun uses the same ammo, so you may want to save up for that. Incidentally, the P90c costs 1 AP less to fire.
Snake Morton, one of the Morton Brothers, uses this gun. Failing that, it can be purchased in San Francisco. Also, some Hubologists that one can encounter outside of San Francisco also carry this weapon. Killing these Hubologists does not decrease (or increase) one's Karma, and will not make the Hubologists in San Francisco turn hostile, even if you kill thousands. The Enclave soldiers that wear combat armor in Navarro also have these equipped.
Originally, the weapon was supposed to belong to the big guns skill and have a bipod.
The real-world counterpart for the G11 is actually a rifle that was developed for the German Bundeswehr but was canceled because of defense budget cuts after the reunification. The E variant is specifically designed for Special Ops., equipped with a low-light scope and a suppressor. Neither of these features are present in the game, which is especially apparent when the G11E is compared to the FN FAL (Night Sight).