Let's suppose you maintain a list with admin email ids. write this on
App. OnStart:
Set(varUser,User())
Set(IsAdmin,LookUp,AdminList,Email = varUser.Email).Email)
This will store email id of the admin in the variable IsAdmin if current logged in user is admin.
In the gallery item code write this
Filter(ListName , varUser.Email = IsAdmin || 'Email Id' = varUSer.Email , and then your combobox conditons as shown in pic.)
What this do is if user is not in the admin list it will only show his/her assignments. If he is admin it will display all the assignments.