First feature is to show the Google calendar view into a tab into Agenda view. This feature does not provide any synchronization (see next feature for this) but only a visibility of Google native calendar into a Dolibarr tab. This list of agenda will be the same for every users so this feature is not usefull if you have different agenda that need different visibility according to users (The next feature may be more interesting if you are in this case).
see the Google calendar into your Dolibarr, go onto Google module setup page, tab "View Google calendar" and:
Choose "Activate this tool" to Yes.