Hello
You are missunderstanding the meaning for this option
Gateway may be used as source device for object copy
(available for B2D stores)
A device with this option selected may replace any device of the same device tag. If the original device is not available for an object copyA copy of a specific object version that is created during an object copy session or a backup session with object mirroring. session, Data Protector automatically selects an alternative device of the same type, located in the same library.
Default: not selected
If you want to select which gateway is used during copy job is must be done from copy job specification into Source devices tab.
Anyway I don't think so that specific could be the issue "Our object copies are now taking a long time" maybe you must check different topics about why performance has descreased, data size, concurrency, number of objects,.......
Best Regards