I know for sure that i cannot use GNU libs with UE4 for client, but what about server-only?
I want to use C++ and UE4 Core/CoreUObject for server side programs, for example for LoginServer and LobbyServer etc…, but i would also like to use MySQL C++ connector with GNU license…
Is it possible? if i use my programs only on server without selling it? Or i am not allowed to do that and i should find another Database for server (or use ODBC proxy)?
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.