In case if someone is still struggling with this, here's how you do Gradient Adaption in Fluent 2019:

Create 1 or more register(s) by following steps 2 & 3
Go to Domain -> Adapt -> Refine/Coarsen
In the 'Adaption Controls' dialog box, click on "Cell Registers" -> New -> Select the type of register you want to have and follow the required steps
Now go back to Domain -> Adapt -> Refine/Coarsen -> Cell Registers -> New -> Field Variable
Under the "Derivative Option", Choose 'Gradient'
Select the type of field variable you want. e.g.: Pressure, Static Pressure
Click 'compute' to see the Max and Min values of the field variable you selected
Choose the type of refinement threshold in "Type". e.g. Cells More Than
If you chose say, 'Cells More Than', provide the threshold value in the 'Cells having value more than' box
Give it a name and now click 'Save/Display' to see the cells that meet the criterion, and then hit 'Save' if you are satisfied. Then hit 'Close'
In the 'Adaption Controls' dialog box, in the "Refinement Criterion", from the drop down choose the gradient refinement criteria you just made
Then hit 'Ok' to mark or hit 'Adapt' to adapt the cells

You can follow similar steps for coarsening the mesh as well.

I hope this has helped someone.