Commercially Supported GPL Themes
WordPress.org maintains a directory of Commercially Supported GPL Themes. Let me quote what they say about those themes...
While our directory is full of fantastic themes, sometimes people want
to use something that they know has support behind it, and don't mind
paying for that. Contrary to popular belief, GPL doesn't say that
everything must be zero-cost, just that when you receive the software
or theme that it not restrict your freedoms in how you use it.
With that in mind, here are a collection of folks who provide GPL
themes with extra paid services available around them. Some of them
you may pay for access, some of them are membership sites, some may
give you the theme for zero-cost and just charge for support. What
they all have in common is people behind them who support open source,
WordPress, and its GPL license.