multiple checkboxes to be selectedfrom gallery and it should update list item based on checkbox selections

[Veblitz]

Kindly help me with below criteria

1)When multiple checkbox  is selected from gallery and Main value  is chooses from dropdown then it should update the selected records in SharePoint list SRType main and incase if the dropdown is choose backup then value should be updated in SRType field  as  backup

2)Alternatively, if the checkboxes are unselected it should clear the values respectively from SharePoint SRType values

Below are the steps and I need help on the formula for checking multiple checkbox values and update in SharePoint list column 

List Name: Shortlisted

Name(person field)     SRType(choice dropdown values Main, Backup)

John

Jim

Mathew 

Gallery(PowerApps)

1)Items property for gallery SharePoint list shortlisted

2)Dropdown item value based on shortlisted SharePoint list where column SRType field dropdown Main, backup values