I used to cycle Kelvin Walkway from Maryhill through to Kelvingrove park. I had a 300 and then a 500 Lumen light. More than enough to see everything.
Depends where you are starting on GWR. At 6am it's still quiet and I have cycled Anniesland but by 7.30 it's getting busy. Only good thing is the bus/ bike lane but again a strong back light is worth it.