No Brooms?

GURPS: Magic allows for items to be enchanted with Flight and  Hawk Flight.

Flight: Item
Staff, wand, or jewelry. Usable only by mages; only affects the wearer. Energy cost to create: 2,500.

Hawk Flight:Item
Staff, wand, or jewelry. Usable only by mages; only affects the wearer. Energy cost to create: 3,000.

A broom falls within the definition of what can be used as a staff, but it's interesting that it's not mentioned specifically.

UPDATE:

As with so many things, GURPS is counterintuitive.

The spell you use to make a witch's broom is Flying Carpet.

"Transforms a carpet (or any other object one could reasonably stand on or in – cauldron, chair, towel, broom, etc.) into a flying vehicle."