Flat side GGs work very well as a pizza/baking steel. You can get really good leopard spots, but you have to be careful not to overdo it.
GGs flat heat up a LOT faster than a stone or steel, which is good. GGs cook the bottom much faster than a stone -- probably similar to how a steel would work on a grill. That can be good or bad.
The steel really shines in your kitchen oven. Your kitchen oven just can't get hot enough to do pizza, so the steel gives you extra browning of the bottom as compared to the stone. Because hot metal transfers heat energy into the bottom of your pie much faster than a stone does. So in your kitchen, hot metal below and hot broiler above is a great way to get the top and bottom both cooked right and at the same time.
On a higher temp grill, though, a steel can give you too much/too fast cooking of the underside as compared to the top. A stone transfers heat more gradually and slowly. Which is why the bottom of a pie in a 900F Neapolitan oven doesn't immediately incinerate.
My gasser pizza setup is a Kettle Pizza Gas Pro over flatside GGs. The Gas Pro is a stainless steel lid that sits on the grates and helps cook the top. Kettle Pizza recommends using a stone, but that takes a very long time to preheat. And I still find the bottom tends to be undercooked. So I use the GGs flatside instead of the stone.
To avoid over-cooking the bottom, I will put the pie on a tin pizza pan at the beginning and/or end of the cook. I also might turn the burners down a bit. The trick is to get the top and bottom both cooked right at the same time.
Like Rich notes above, I can get good results just using the regular grates. I'll start the pie off on a pizza pan on the regular grates while the lid of the Gas Pro cooks the top. Then I slide the partly cooked pie off the pan onto the grates halfway through. Which lets the bottom brown up while the top finishes cooking.
