Can clients provide or use an existing Gateway Hub device on site?

Yes they can, depending on the (courtesy of the) gateway owner. A gateway typically sends sensor  payloads to a designated LoRaWAN cloud server. From their payloads are forwarded to corresponding receiving parties and databases. When a third party LoRaWAN server forwards Factorylab payloads to the Factorylab database, then the desired connection has been established.

Though technically possible we rarely see this happening. It’s often easier (and cheaper) to just install an own (low cost) gateway and maintain independent system security.