I had the same problem with the freezer sheles being too small in my GE Profile side by side. The shelves would simply collapse down with any weight on them.
After measurng the gap, I determined that the shelves were approximately 7/16" too narrow.
The fix I did was simple, effective, and very cheap.
I bought a roll of clear flexible polyethylene tubing at Lowes for about $5.The tubing is 3/8" outer diameter, 1/4" inner diameter (see picture below).
I cut pieces to fit along the side rails of the shelf, and then sliced the tube pieces down their length with a utility knife. I wrapped the pieces around the outside rails on each side of the shelf. It took a little work, but I got the tube pieces attached.
In a few spots, I did a double wrap of tubing.
The shelves fit tightly now and can hold significant weight without collapsing. Plus, they don't look too bad.
I hope GE fixes this simple problem in the future.