Absolute beginner with GT.M coming from Oracle/relational DBs

Hi,
I'd like to try to familiarize myself with GT.M but I don't know where
to start. I read a few lines of theory regarding hierarchical database
systems, but now would like to try to implement simple things, using
this engine. Is there a web page somewhere explaining you and telling
you how to create entries ? How to retrieve, update them etc. ? I'm
confused because I do not understand the difference between GT.M
and M[umps]: is it the same ?
If there are no tables and rows in a hierarchical DBMS, what are the
equivalent concepts in GT.M ? I read things regarding the "difficulty
to implement many-to-many relationships": could someone give me
an example of working this around with GT.M ?
In advance, thanks so much !
SR