The route: Rishikesh → Chamba (60) → Tehri (85) → Uttarkashi (155) → Harsil (240) → Gangotri (265). The Bhagirathi gorge above Uttarkashi is, for our money, the finest stretch of motorcycle road in the state.
Realistic timing: three days. Day 1 to Uttarkashi. Day 2 up to Gangotri and back to Harsil — an old deodar-forest apple village where almost nobody stays and everybody should. Day 3 back.
Which bike: Himalayan. The road above Harsil deteriorates and the altitude gain is real — Gangotri sits at 3,100 m.
Road conditions: good to Uttarkashi, excellent to Harsil, mixed above it. Landslide-prone in the gorge sections after rain.
Permits: yes — Gangotri is a Chardham shrine, registration mandatory and free. The Gaumukh trek beyond Gangotri requires a separate forest permit with a daily cap — a different thing entirely, worth knowing if you're headed there too.
Fuel: fill at Uttarkashi. Harsil has limited supply and Gangotri has none.
Where to stay: Uttarkashi on the way; Harsil on the way back if you possibly can.
Season: temple opens late April, closes early November. May–June and September–October.