Link GPO dialog has a column with an 'Add' checkbox for each link. The checkbox is selected by default.
What is the meaning of the Add checkbox and what happens if you unselect it?
The only mentioning of the Add checkbox in the GPOAdmin User Guide is the following:
In the right pane, ensure that the Add check box is selected for the GPOs you want to link.
When the 'Add' checkbox is selected, the corresponding link is created.
If you unselect the 'Add' checkbox, the correspondinng link is deleted, or is not created if it did not exist.