The HK G36, or Heckler & Koch G36, is a German assault rifle designed by guns manufactruer Heckler & Koch. Apparently, it was designed in the early 1990s as a replacement for the heavier 7.62mm G3 battle rifle. It was accepted into service with the Bundeswehr in 1997, replacing the G3. It is gas-operated and feeds from a 30-round detachable box magazine or 100-round C-Mag drum magazine.
In the game, the HK G36 was used by the old guard model, which was in use until v0.6. The new guard model uses instead the FN P90, or rather FN Herstal P90, a Belgium personal defense weapon designed by guns manufactruer FN Herstal, which is more fitting as it's not only comptact, but just as powerful.