You could base your Gallery on a collection, adding a field for SortOrder and a field for a 'New' flag. When you hit the Add Task button add (patch) a new row to the collection and use a Sort Order that will ensure it appears at the bottom (if you want this so act like Access where new records are added at the bottom).
Make the controls that show the fields in your Collection text boxes (or dropdowns or whatever is required for data entry, just no labels as they can't be edited). For the existing records put these in DisplayMode = View (unless you want them to be editable).
For the new row the controls will need to be in DisplayMode = Edit (you can make this conditional on your 'New' flag). You may also want a 'save' button that displays on the Row when in Edit mode. In the OnSelect of the Save, Patch the entered values back to your datasource then rebuild the collection on which the Gallery is based.
However, that is a lot of effort to build and maintain, so I would not go down that route, rather, when the user hits 'Add New Task' just show the relevant fields on the screen (not in the gallery).
Trying to make one system (Power Apps) look/work like another system (Access) is a lot of unnecessary work IMO, accepting that each system has its own variations and limitation and working to that will make life much easier.