Unreal’s license is explicitly incompatible with GPL.
I quote:
" … 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. … "
Source: https://www.unrealengine.com/en-US/eula
Section 1) B) ii)