The HDL source code is just that, source code: as such, it can be licensed with the GPL or any licence used in the software world. Combining GPL and proprietary circuits into the same slice of silicon, however, is still a relatively unexplored and grey area, so check with your lawyer first.
At the beginning you hinted that FPGAs can have a deep impact on hobbyists and education. How?
Because we have reached the point where FPGAs are big and cheap enough that an FPGA development board, containing one FPGA, all kind of connectors (USB, VGA, FireWire, serial…), some RAM and other goodies, not to mention the development software, can cost as little as a couple of hundred pounds! In other words many hobbyists and public school are finally able to make their custom hardware or set up a Digital Hardware Design Lab or course on a shoestring budget: what are you waiting for?
Wonderful! Where do I get a development board? Can you recommend some model?
Technology and prices move very fast, and the choice of the right board depends a lot on which peripherals you actually need, so it wouldn't make sense to name any model: a quick internet search for "FPGA development board" will return plenty of possibilities.
Is there some online community or portal where I can start learning FPGA design?
Of course there is. www.fpgacentral.com or www.opencores.org are good starting points: the latter also hosts hundreds of open source projects.
OK, so FPGA design can be both affordable and Open Source as Linux: is there any other point of contact?
Yes, and a very interesting one at that: the latest generations of FPGAs are so big that they can embed whole microprocessors like ARM inside them and still leave plenty of space for your own custom functions. Consequently, you can run Linux inside an FPGA! How to do it and what is happening in this space, however, is a topic for another article!