Are there check-in points or registers along the trail?

You will need to register with Parks Canada for backcountry permits for those sections in the National Parks, and with Peter Lougheed Provincial Park for backcountry permits in that park. Outside of the Parks, there are a few informal trail registers along the GDT (e.g. Baril Creek, White Goat Wilderness Area, the summit cairn on the highest point on the GDT) but most other areas don’t have check-in points and don’t require registration. It is always a good idea to tell someone where you plan to go and when you plan to return.