As Class G airspace is "uncontrolled" there is no requirement/ability for an ATC "Clearance". Thus in theory one would be free to fly without talking to anyone beyond that required for VFR flights into/near airfields in Class G airspace.
Canada requires a Flight Plan be filed.
The USA does not....just requires the aircraft and pilot be legal for IFR flight. Procedural reports to other aircraft are required in addition to normal traffic reports.