Gcal is the name of a custom app for Smartthings. Whether you use Gcal or IFTTT you are going to create a virtual switch in Smartthings that gets turned on and off based on the start of an event in the Google calendar. Candle lighting events mean the start of shabbos and yom tov and havdalah events signify the end. You then program the mode to follow the switch's state.
Not sure why you want to avoid GCal and IFTTT, the HebCal integration is also reliant on the cloud (HebCal and Smartthings). And honestly, the way the official app queries at 8 am exactly has caused the integration to be unreliable (this could be for several reasons). You are much better off using Google calendar which is very reliable.
Smartthings is not supported in Israel. I agree there is likely an issue with the zip code lookup in that country. As far as the push notification, this feature is broken in the version of the smart app provided in the Smartthings store but I have corrected it a custom smart app that I use.
I would suggest that you instead import the HebCal data into a Google calendar and use either IFTTT or GCal Search to trigger a mode change based on "candle lighting" and "Havdalah".
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