Is GRE a requirement for most of the courses I am interested in, and is it mandatory or optional in Canada/USA for a Master's?

It depends on the university and program.

For most master's programs in the USA, the GRE is often required, but some universities may list it as optional. In Canada, the GRE is generally optional for many programs, though certain courses may still require it.

You can target programs that either require or waive the GRE based on your preferences.