A gallery of people profiles will automatically list the profiles alphabetically by last name.  There are two things that can affect this order:

Reverse Display Name

The alphabetical sort order keys off of the Reverse Display Name field. If this field is left blank, the gallery will not sort properly.

example reverse display name field

 

Override and Sort Order

Override Alphabetical Sort: check this option if the person should have priority placement in gallery view.  This option is often used for department chairs and center directors. It will place them ahead of everyone else in the gallery view. All other profiles will display alphabetically by last name after the prioritized profiles.

Sort Order: allows you to specify the order when multiple profiles have been given priority with the override alphabetical sort option.

example override and sort order fields