Lots of conflicting advice here. Mine is NEVER use those poly sleeves. To see the prints properly, they must be removed and for a gallery owner that is time consuming and unnecessary. Most will not bother to remove the sleeves--and hence, the work will not be properly seen. In that situation, there should be nothing between the print and the viewer.
Mounted and matted is best unless the prints are very large, in which case unmounted is fine.
And has been said: no slipsheets. Make it easy for them.
And no white gloves. They are insulting. If you feel a gallery does not know how to handle prints, why would you show them your work?