Gantry 4 and our RT extensions use a very old version of MooTools that we cannot change. As the message states...
This site overrides Array.from() with an implementation that doesn't support iterables, which could cause Google Maps JavaScript API v3 to not work correctly.
... and it is MooTools that is implementing that override. Therefore that google map API v3 is not going to work as long as MooTools is loaded. To prevent that happening you your need to disable all RT extensions and probobably turn off some other options in the template (those that might cause mootools to be loaded eg "fixed footer") - this would include Roknavmenu, rokbox, rokstories, etc. Once you've done that there's not going to be a lot left and perhaps this may be the thing that leads you to conclude that you should use a new template.
Gantry 5 and it's templates use jQuery rather than Mootools as do it's particles (which replace many of the RT extensions). Gantry 4 has not been further developed for a long time now - all the development effort has gone into Gantry 5.
So, sorry, I don't think I can help any further. If you want to use modern software (like google api v3) then perhaps the time has come to move on to a modern framework and theme.
Regards, Mark.