I am assuming that the checkbox is working fine even for the group filters, it is only selecting the value currently getting displayed in the gallery.
To perform a bulk update:
Submit Button -> OnSelect -> ForAll(Gallery1.AllItems, Patch('Table Name',Lookup('Table Name','Sender Serial Number' = Label1.Text),{UserColumn:User().Email, Date: Today(), Flag: Checkbox1.Value}))
This is a sample snippet, which you can modify based on your data source and controls.
Here, Table Name is the name of table to perform operation, 'Sender Serial Number' is the column used to identify the record. Label1.Text is the name of control which holds 'Sender Serial Number' inside the gallery. Inside curly brackets { }, you need to pass the columns you want to update.
Hope this Helps!
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