It's typically too low for boats by mid-July. Watch for downed trees. As the Gallatin churns downstream, it grows increasingly difficult. Big Sky to Greek Creek features tight turns and challenging rapids as the river picks up volume. There's a particularly challenging spot near where Portal Creek spills in.
Intermediate rafters can handle it, but it's on the margin for canoeists.
The real heavy-duty Gallatin whitewater lies between Greek Creek and the Squaw Creek Bridge. Not long after Greek Creek, the canyon narrows and the river changes. The turns get tighter, the river grows rockier and the whitewater becomes more continuous. It gets hard to find eddies where it's easy to pull out and bail or take a break. Below the Cascade Creek Bridge the whitewater is nearly continuous, with big rocks and big water in the narrowly constricted river.