The tunnel is 5km long. I measured the south portal at an elevation of 2751m and the north portal at 2640m, giving a 2% gradient. The tunnel is not dead straight as the map suggests, there are some gentle curves.
The tunnel has one lane of traffic in each direction, a rideable margin on both sides (though with raised ironworks), and a narrow raised pedestrian footway on each side. There are a handful of laybys big enough to accommodate trucks, and frequent refuges for pedestrians. In fact I think there is a parallel narrower tunnel the whole way, perhaps for emergency access, which you can cross into at various points.
There is some lighting throughout but it is very dim, you need good lights to see and be seen. There are signs of firefighting equipment. There is some forced ventilation, the exhaust fumes were not too bad but there was a lot of dust in the air. The road surface is concrete. There were a few puddles of water in places.
Tajik drivers overtake inside the tunnel and mostly don't seem to obey the 40 km/h speed limit.
If you don't want to ride through the tunnel it shouldn't be difficult to hail a passing van or truck and get a lift.