Right click the enemies and name them all the same (I.e. wave1 for the enemies to spawn at the first trigger) then set them to spawn at start = no.
Now place a trigger zone somewhere on the map the player will reach first (resize it if needed) and set the name of the first wave enemies (in my example; wave1) in the ifused field of that zone.
Repeat the process using a different name for the next wave
(You will need to put the triggers in different areas or do some scripting to change when the triggers are activated if you want them to be in the same area - for a defend the position type map)
The main issue with doing this in GG is that you have to set up every enemy in the editor first (although its easy enough to extract and place multiples it will drop frame rate a lot) - there is no clone or create entity command via script
life\'s one big game
windows vista ultimate
i5 @3.3ghz, 4gb ram, geforce gtx460, directx 11