Hi dddvvv, and welcome to the Blackvoxel forum
Unfortunately, what you are planing is not possible.
GPL v3 license means that any part of any derived work must be free software. Doing proprietary artwork or game content in order to turn a free software into a proprietary software would be a license infringement.
The GPL license is a "non permissive" and "viral" license. Any derived work must keep the freedom of the original work. With GPL, even externally linked code, content and assets are "contaminated" and must be released under a compatible free license.
To make it simple, if anyone want to use any Blackvoxel code and/or content to make another game, the price to pay is to make the game entirely free software. We won't tolerate any other scheme.
With Blackvoxel, we turned years of hard (and costly) work to free software, in return we expect any derived work to do the same. So we (and the free software community) can use derived work to improve the original work and other free software.
You say your goals are about personal development and to build knowledge with pleasure. If so, doing free software is the way.
About Blackvoxel, be aware that the game remains unfinished : a lot of features are planed. In particular, missions including puzzles are planed on the Blackvoxel roadmap since a long time. So it will end up in the game someday...
In the hope we have answered to your questions
The Blackvoxel Team.