Once you've dealt with 2, (including the 5 or 6 cyborgs who like to stand on the mountain W of the depression), your next target should be... 3!
Real simple. Pick your bombers again, and 1 STUPIDVTOL, and sail them up the right side of the map to this area. Bomb the SAMS and the CB tower (the angel batteries should be long dead). Simple.
And now with 3 down, we move onto.. four!
Same as with 2, send in some STUPIDs, and watch your artillery bring it down... and kick off the "goshdernit".
As soon as the NEXUS command center is taken out, NEXUS "activates" itself in Gamma. To describe "NEXUS activation"... think Borg, only with a sense of humor.
In one snap of the fingers, this base completely converts to NEXUS control. They're gone, Jim.
Luckily, for all us cold, calculating Project Commanders, we can immediately get over the loss of the people here, and revel in the realization that Gamma isn't very well equipped. Other than those "NEXUS" towers, most of what they have is stuff you've had since early BETA.
And the only NEXUS help they will have is 4 hovertanks SE of the base, and anything from base #4 that got out alive.
Furthermore, your timer gets a nice, healthy boost to it, allowing for LOTS of power-leeching in this stage.
Time to send the Cavalry to Gamma.. though not in the way they had originally hoped.
Note: Gamma base is rather poor, but they have a fair # of AA guns. Be sure to take out the 4 NEXUS hovertanks SE of the base before going after the base proper (there is a heavy-SAM among the tanks), and when you finally do go after the base, do something that I've never advocated before, and never will again:
Set your Bomber fleet to Retreat at Medium.
Against SAMs, turning and running is a waste, as you get chased down. But against AA guns, which operate on "small damage, high rate" methods, having a plane turn around when it gets to 75% will be easily enough for it to run away alive.
Give some queue-ed orders to take out the AA pits first. The Assault Gun towers are of little concern.
Whatever you do, if for some reason you have land units in this stage, do NOT send them in. Those spiky-shaped towers are the ones that turned Gamma into a NEXUS base. you send in tanks, they will turn them too (slowly mind you, but losing nothing > losing small #s).
The purple circles in the Gamma picture are the structures that have the Research modules in them. After you are done blowing up the things that can fire back, take these out so that your research teams can get to work again. They Cyborg factory can be left to draw the stage out.