Hi,
If I assume that the field "Nature" is a Choice column in your Dataverse table, I suggest you to use RadioGroup instead of multiples Checkboxes, where Items are simply :
Choices([NATURE OPTION SET])
This formula will retrieve all possible options for your Choice column defined in Dataverse and will avoid having multiples checkboxes in your app that will do the same thing.
After this, in the Items property of your gallery, use this :
Filter([SOURCE] , IsBlank([RADIO_COMP].Selected ) Or [NATURE FIELD] = [RADIO_COMP].Selected.Value)
Regards,