Yes, you can use this plugin for free also within a commercial context. The only requirement beside following the GPL 2.0-licence is that the attribution links to the plugin and the licence conditions of the used maps don´t get removed from each map shown through the plugin on your website.
Since v2.5 the plugin also supports the usage of Google Maps as basemaps. Please be aware, that to stay in accordance with the Google Terms of Usage you have to use an API key for commercial usage. Please see http://www.mapsmarker.com/google-maps-api-key for more details.
If you are using this plugin within a commercial context, please also consider making a donation, which allows the plugin´s author to spend more time developing, maintaining and supporting this plugin.
Posted in: General