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In simple terms, the General Public Licence (GPL) provides you with the right to personal use of the open source but requires you to make your own work available under the GPL if you distribute it.
The following simplified interpretation may help answer the most common questions though there may be variations depending on the exact circumstances. The GPL itself should be used as the legal definition of what is allowed.
This is exactly what the GPL is for. You are free to use the open source software in any way you wish in a non-commercial environment so long as you do not distribute it to others.
This is also fine so long as all the development is done by you but remember that open source products come with no warranty or support. Use of external developers counts as distribution and this would require that your own developments are also released under the terms of the GPL.
You may do the development work using the GPL version. When you come to distribute the product, you must release your own software under the terms of the GPL. The end user is also bound by the terms of the GPL.
Regardless of any implications of the GPL, transfer of application software between the two product formats is against the terms of the commercial OpenQM licence, whether you sell or give away your developments. You may, however, move your uncompiled source between the two environments.
This is not related to the GPL in any way and is the best approach as you will be using fully supported software throughout.