Gunflint Trail (north shore/grand marais) TT free camping?

I suggest that you call the Gunflint Ranger Station and ask them specifically.
Ask for the Assistant Ranger for Recreation. The phone number for the Gunflint Ranger Station is (218) 387-1750.

That said, the Gunflint trail is a ribbon of land that isn't very wide, since it is a dead end paved road (and National Scenic Byway) that cuts up through the Boundary Waters Canoe Area Wilderness. Link to Gunflint Trail National Scenic Byway

There are several resorts and several USFS campgrounds but these of course all charge a fee. As robbiesgram has said, there may be some short closed road spurs that you might be able to park along. Back in 1999, there was a huge windstorm which blew down and snapped off hundreds of thousands of acres of trees in the BWCA and also along the Gunflint Trail. There were a lot of salvage timber sales to harvest some of the blown down timber, so there may be some old roads going off the Gunflint trail. But the USFS generally closes those roads and installs berms just off the main road to prevent people from using them. They were built as temporary roads just to access the timber, and are not really available for public motorized travel.

While you are up there, be sure to pay a visit to the Chik-Wauk Museum. It will provide you with a glimpse of what the area was like in the past, including the Anishinabe people, the logging era, and the fishing resort era of the 1930's to 1960's. The museum itself is housed in an old stone resort/lodge that did a booming business back in the fishing resort era. Link to Chik-Wauk museum