I wasn't actually the one who designed those gussets, but we have a couple of methods for lightening things on 100.
First, the one you saw in the photo. Basically, we draw out all our critical features, and cut out everything but some "spokes" that run between critical holes and features. It is meant to take load that goes from one feature to another in a straight line like Adam described. Then, we add safety circles around all of the critical features and put in fillets. Perhaps not the most efficient method, but it does look great. A number of west coast teams pocket in ways like this. It also takes a while, so beware.
Second, just plain old holes. Sometimes, when we've got a big long member that we want lightened, we take it over to the drill press and just cut out a bunch of huge holes with a hole saw. If you do them regularly, they don't look too bad, and more importantly, they're a lot faster to do then milling. This is a pretty inefficient lightening method on the other hand. It may not be the strongest either (although I've never seen a break because of the lightening holes on a part like that). This method is good for low resource teams too, as you don't need a mill. We did this on our first iteration climber arms:
Third, don't forget about plain old pockets. For large plates that need to be thick for some reason, or tubing we want lightened, we can set it up in the CNC, and basically take all the material not around the cutout or critical features to about .060". This saves a lot of weight, is faster to design and machine then the first option, and looks real slick (milled alu finish for the win!). Check out this for an example:
I have to remind you,
the number one way to reduce weight is to reduce material thickness. If you've got a frame made out of 1/8" 2x1 and you change it to 1/16," you're basically cutting its weight in half. We aren't too gentle to our frames and we've never had a problem with 1/16" thick tubing. Also, very few plates on a robot need to be out of 1/4" plate, or even 1/8." Our bellypan was all 1/16" alu, and is perfectly strong. We probably could have even gone with 1/16" thick gussets instead of 1/8" too.
Most material isn't doing much. Lightening removes that material. Structure you're lightening patterns as such.