I built my gMax from scratch as a 1.5 so part-wise I didn't keep track of what is different between 1.0 and 1.5.
On the base, the two shorter extrusions are end tapped on both ends and have bolts placed in each end. The heads of the bolts are slid into the longer extrusions to form the frame. The bolt heads acts as t-nuts. The longer extrusions have access holes drilled into them to allow a hex key through to tighten the bolts. Once everything is tightened the frame is very rigid.
The same approach goes for the upright extrusions connecting to the top bar. The upright extrusions are only end tapped on the one end that connects to the top bar, though. That assembly is then connected to the frame base using the L brackets and t-slot nuts.
Technically, one could drill access holes where the uprights should be, end tap both ends of the uprights, and use bolts to secure the uprights to the base as well as to the top bar. I don't know how / if it would affect rigidity of the uprights, though. I think part of the reason gCreate uses the L brackets for the uprights is because 80/20 can do all of the machining they need this way -- 80/20 only does access hole drilling at the offsets necessary for connecting the extrusions at the ends of the cuts as far as I know.