Umm Qais Museum is a small but useful stop inside the ancient Gadara site. Its displays help explain the basalt city, Roman remains, and local finds before you step back outside to the terraces and long views over the Jordan Valley.
Do not expect the scale of Amman's archaeological museums. The point is context: a short indoor pause that makes the theatre, colonnaded streets, tombs, and black-stone houses around Umm Qais easier to read.