Comment 1 for bug 1456712

Gradient editing is done on canvas with the Gradient Tool. You can add and remove stops right on canvas with Ctrl+Alt+Click along the gradient line (or just double click on it to Add a stop). If you need a dialog you can re-enable it in the Preferences for the Gradient Tool (double click the tool's icon), but this isn't recommended as it has issues.

Inkscape automatically cleans up unused gradients, so if it is not assigned to an object, it gets removed.