I'm don't think the Gantt chart view is the best choice to discover this information. The only timescaled view is the graphic on the right. Certainly collapsing the summary tasks (assuming you have created summaries for each of the runs) will show you the gap.

I suggest looking at the Task Usage view (with the Project Summary tasks visible) zoom out to weeks or months and look for the gaps.

The Resource Usage view with Remaining Availability added to the timescaled side (right) will also show you when your crew is available for other work.