Will she have to scan the MRZ on her visa page, or the bio page?

Your girlfriend will need to scan the MRZ page as that is the way the computer will read the information, using the chip on that page.

Also, will the gate even work for her given her previous immigration issues?

There is a possibility she will be sent to the passport control officers, there is no harm trying to use the gates.

Will she have to scan the MRZ on her visa page, or the bio page?

Your girlfriend will need to scan the MRZ bio page as that is the way the computer will read the information, using the chip on that page.

Also, will the gate even work for her given her previous immigration issues?

There is a possibility she will be sent to the passport control officers, there is no harm trying to use the gates.