There is a way to license it together, and to keep the licenses separate, which is particularly useful when there is artwork or fonts with licenses incompatible to GPLv3.
The GPL FAQ includes this statement: "Keep in mind that some programs, particularly video games, can have artwork/audio that is licensed separately from the underlying GPLed game. In such cases, the license on the artwork/audio would dictate the terms under which video/streaming may occur." This means that it is recognized by FSF, that there may be content within software under GPL that has a different license and GPL does not extend to these parts.
The CC Wiki states: "When someone creates an adaptation of a BY-SA licensed work and includes it in a GPLv3-licensed project, both licenses apply and downstream users must comply with both licenses. However, Section 2(a)(5)(B) of BY-SA 4.0 allows anyone who receives the adapted material downstream to satisfy the conditions of both BY-SA and GPLv3 (i.e. attribution and ShareAlike) in the manner dictated by the GPLv3."
You may, however, also license the artwork and fonts under GPL. There is an opinion of FSF on this in Software Engineering Stack Exchange and what you can do to additionally cover trademark rights and other issues, however (unfortunately) they just don't touch the aspect of license-incompatibility.