MQTT is a protocol that several IoT applications do can already talk to each other, openHAB being among them.
What you will need is that your Grove or Tart software (to read the sensors) can send sensor output in MQTT form to your openHAB server. If they cannot, you can either run a script on that gateway Pi (be it Python or any other script language) to do that for your, i.e. extract the sensor values and forward them to the openHAB server encapsulated in MQTT. Here’s an example that reads input from a Pi’s GPIO pins and forwards this in MQTT.
A third option would be to use the HTTP binding (= a software module running on the openHAB home server) to pull data from that openHAB server off a webserver running on your gateway Pi (I assume your sensor software will have a web interface).
But as you can see, there’s more than one solution, and it all depends on the software you want to run on the gateway.
Well, there’s no off-the-shelf dashboard, but you can tailor how your UI does look like. As a starting point, a demo set of items and a matching demo UI are provided as part of the default installation, see items and sitemap directories. Further docs on UI config are here.