['97ventureowner]

The market has shifted in the past few years away from minivans, as evidenced by the lack of any in GM's US lineup. Crossovers have taken their place. And SUVs have also seen declining sales from both high gas prices of recent years and the fear they may go up again, and the fact that crossovers have entered the market, siphoning some of their sales. GM needs to streamline their truck lines in a way to keep them profitable as it offers too many models and sub models of each.
If they must keep the GMC brand then perhaps move the commercial line like you stated entirely to GMC and include the full sized vans, leaving the crossovers, SUVs, and smaller pickups to Chevy. That could save space at the dealership and spinoff the GMC division to separate locations in cities and towns where they set up shop. GMC is usually paired with Pontiac or another brand at the dealer, but by moving it into a separate location, the staff at the dealer can focus entirely on the GMC brand which can be helpful to their commercial customers as their facilities could be setup entirely to sell and service these vehicles with no "non-truck" vehicles present.
To save even more money ,GM could determine which areas would receive a GMC dealer as not all areas could support a GMC-only dealership. Keep it to larger urban areas where there are a lot of business customers, or between two large population areas, rather than have one in each nearby town, duplicating services. This could work well with GM's current plan of cutting back on dealerships, and placing them more strategically in population centers.