At the moment your are editing any requirement (steps, points awards or deducts and rank requirements) you are able to choose the event “Mark as completed” as you can see here:

After choose the event “Mark as completed”, a new line of settings will appear letting you decide the way the controls should be displayed (as a checkbox or as a button) and its position (before or after the requirement text):

By default, when you save the requirement with an empty label, GamiPress will generate one using the requirement configuration like the event name and its limitation settings:

Of course, you can edit this label with any text of your choice:

And this custom label is the text that will be displayed at frontend to your users:

In addition, you can configure multiples requirements using the “Mark as completed” event and configure them to being displayed in different ways. There is an example of an achievement with 2 requirements with the event “Mark as completed” and one displays a checkbox before the label and the other displays a button after the label: