MT data is stored in the Meraki cloud and available through the dashboard for 1 year and you can view up to any 30 days at a time. As for the gap, if you mean in coverage then make sure that an MV camera or MR access point (Wifi5 wave2 or newer) is nearby and in the same dashboard network. The sensor will tell you the signal strength for any MRs or MVs it can see and the last time it communicated with them.
@Akash_5321 what @Greenberet said is correct about the offline storage, in the event your network is down and the MT sensors have no Dashboard connectivity, they can cache reading for 5 days.
MT sensors are always sensing and report into Dashboard every 20 minutes, or immediately upon violations or a state change with customizable alerting you can configure. This default of 20 minutes regardless of state changes is in order to help guarantee the long battery life. I've had my own MT20 in my home lab well over a year and the battery is currently at 70%.
Changing the sampling rate is possible on the back-end but is experimental only, it's not supported and could cause suboptimal performance and faster battery drain, so even Meraki Support may not be permitted to adjust. What I mean is the 20 minute default is obviously something that is "configured" but it's a back-end setting not meant to be adjusted.
I would agree the wording in the data sheet "at the default sampling rate" leads one to believe it's adjustable. It's not officially adjustable at this time. Was there some concern about increasing the frequency at which the MT20 checks into Dashboard?