I think you are referring to the GNU General Public License (GPL).
Unfortunately, you are not allowed to combine any part of the Unreal Engine with code under the GPL. The GPL is one of the Non-Compatible Licenses that are mentioned in Section 1(B)(iii) of the EULA.
Please note however, that you are allowed to use code under the GNU Lesser General Public License (LGPL) with the Unreal Engine, so long as you are only dynamically linking to that code. You can see some other examples of permitted and prohibited licenses in Section 1(B)(iii) of the EULA.