Not MSI is the biggest supplier for gaming notebooks, but Asus. At least Asus now claims the title for themselves. The advantage is particularly big for laptops with Nvidia GeForce GTX GPUs.
http://www.notebookcheck.net/Notebooks-Asus-is-the-biggest-supplier-of-Gaming-Notebooks.172041.0.html
I would like to have a full survey of the gaming netbook market shares. Asus has the very good taste of topping regularly the market shares AND topping the "build quality" benchmarks, i.e : Asus tend to have less problems than other brands.
Other brands like Razer have very poor feedback ("4th replacement in a year") but I do not know whether this should be alarming or not. If they sell very few laptops, it's an issue that there are so many complaints. If they sell tons of them, you will always have someone complaining
The data can be misinterpreted. MSI pretty much only sells higher end gaming laptops. Asus even sells ultrabooks with a discrete 840M cpu for example which can be counted as a gaming laptop as well if the main criteria is a Geforce or Radeon CPU. In the end I dont see a lot of Asus or MSI gaming laptops in general in the wild.