There is no easy way to change the direction of gravity, at least not for characters that have to stay upright.
Anyways, a somewhat easy workaround is to rotate the map around a player. Just build the map as blueprint and then rotate the actor (alternativley just attach everything to a parent after placing it, and then rotate the parent). This way the gravity is still z-up, but all physics related things kinda fake a change in gravity direction.
The next best solution is to rewrite everything to work with an arbitrary gravity vector, or alternatively wait for epic to implement it, since they have it on their roadmap 