Could black holes be used as gateways for intergalactic travel?

 Theoretically it's possible. From solution of general relativity it is clear that black hole connects two different points (may be long enough distance) via a wormhole. But in reality that wormhole is very unstable unless it gets exotic matter.

In current technology,it is not possible the inter-galactic distance travel. As each galaxy is far away from another, we don't have proper theory of it ( travel faster than light).

For a type 3 civilization,it could master the whole galaxy. So they might have some theories that we don't have now.

As space time curvature joints two different points in space by a tunnel (wormhole) , scientist Kip Thorne showed this in Interstellar movie.