hi
the best site for all country's holidays is
http://www.officeholidays.com
i do not have any problem that you refer to about events showing up in GMT.
i have, as before with macOS Sierra, turned on TimeZone Support and it works very well.
to remove an event, just highlight the event and press delete.
to remove a calendar, have the list of calendars showing in the left side list bar, highlight the calendar you want to delete, two-finger click to get the submenu and click delete.
iCloud + calendars:
you can have any other iCloud user be sent an invitation to subscribe to a calendar you made.
you can not however send an invitation to someone for a calendar that is found on a different person's machine or a calendar such as above from officeholidays, which makes sense, since it is not your calendar.
Calendar is really a mature app in macOS and its iCloud integration is really good now.
it hasn't always been that way, but it is quite stable now.
i am in frequently going back in and out and through multiple timezones in asia and adding flights from expedia or air asia and appointments i add myself.
what you need to do to keep it all functioning is to simply:
(1) turn on time zone support
(2) change your macOS calendar manually when you arrive to your new timezone so the appointments that you make while in that time zone change automatically as you are in other time zones
(3) turning on automatic time zone updating in Preferences/Date&Time/TimeZone keeps the system clock up to date (but, as said above, the timezone that calendar uses needs to be changed manually)
(4) you can check to see which time zone you or expedia etc has set an even by clicking on the time itself of the event and by doing that, it opens a small panel that lets you see the time zone that the event was set up in.
