It's true that you can charge for GPL software. It's just not a particularly good strategy. Because invariably, that software will wind up easily accessible on the Internet for free, maybe even with a higher page rank than your own page.
That's why companies typically dual license or charge for support with GPL software.
For an example, see the hilarity of when Sveasoft tried to sell subscription access to GPL linux router firmwares -- it actually worked for a little while when they were the only game in town, but quickly disintegrated. Eventually they just started ripping off OpenWRT and died.
That's why companies typically dual license or charge for support with GPL software.