The intent of this license is to share developement and its benefits so you can copy what you wish from the GPL software but the "output" needs to also be released as GPL. The license clearly states that you are granted the right to make modifications under the terms of GPL
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License
This term doesn't apply to "separated" parts of the program. But the database design isn't separated (like, for example some external library). The database scheme is protected under copyright (as a whole) i don't see any reason why it should not be. I bet if we spoken about copying some part of paid software no one would have such doubts.