Gaps between annotations on a tier can be transformed into annotations on the same tier or on a new tier. To do so, click. In the dialog window, select the (independent) tier for which you wish to transform the gaps into annotations. Then select whether you wish to put the new annotation on the same tier or on a new tier and specify a tier name in the latter case. Also specify the contents of the new annotations: either a specific value, the duration of a gap or no contents.
It is possible to select multiple tiers when creating annotations from gaps. Selecting multiple tiers can be done by holding the CTRL key while clicking other tiers than the one already selected. The SHIFT key can be used in a similar way to select the range of tiers from the one that is selected to the one that is clicked. The gaps created from multiple tiers are periods where each of the selected tiers has no annotation.