The Lambda Gate is an intermediate galaxy gate made of 45 parts.
Consisting of normal boss aliens, it is the Galaxy gate that requires the second lowest number of parts to be completed (the Alpha Gate requires less parts). However, it is the fastest to build because it has so few parts and all spins on it go specifically to getting parts for it and no other Galaxy gate.
The second wave will come after half the first wave has been destroyed. The same thing happens with the third wave after half the second wave has been destroyed.