Hello Alberto,

About Google calendar, iCagenda is not managing importation of events, and has only a add to calendar function.
So, iCagenda won't be the best one for what you want to accomplish. In fact, this is not the main purpose of iCagenda, as others extensions are already specialized for Google Calendar (you will find many ones on the Joomla Extensions Directory, and almost all using the same script library, and so with a similar design).
Maybe one day i will work on such a development, to communicate with Google calendar, but currently i'm focused on other major developments for iCagenda itself

About SP page builder, not sure it works at 100%, but i think i remember a user using it with success (myself, i've never tested SP page builder, so i can't give you a clear answer).

Hope this answers your questions ?

Best regards,
Cyril