The UE4 EULA does not permit use of GPL licensed content in connection with the engine. Specifically:
You may not combine, Distribute, or otherwise use the Licensed Technology with any code or other content which is covered by a license that would directly or indirectly require that all or part of the Licensed Technology be governed under any terms other than those of this Agreement (“Non-Compatible License”). Code or content under the following licenses, for example, are prohibited: GNU General Public License (GPL), Lesser GPL (LGPL) (unless you are merely dynamically linking a shared library), or Creative Commons Attribution-ShareAlike License. Code or content under the following licenses, for example, are allowed: BSD License, MIT License, Microsoft Public License, or Apache License. You may not sublicense the Licensed Technology under a Non-Compatible License.