I’m Epic’s attorney, so I can’t advise you on the interpretation of the GPL.
I will note that the relevant restriction in the UE EULA is as follows:
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”).
This means that as long as your use of GPL-licensed code doesn’t violate that provision, it is OK with Epic. This becomes a fact specific question, but a possible example of such use would be a server that uses some GPL-licensed code (but does not use UE) and a client that uses no GPL-licensed code at all, but does use UE.