Right now I live outside of Israel and I use the Gregorian calendar , just to be sure about it , is it a sin that I am not currently based on the Hebrew ?
Thank you for your question.
The difference between the Gregorian calendar and the Hebrew one, is that is the Gregorian calendar is based on the sun, whereas the dates of the Jewish holidays, are based on the lunar calendar, i.e. Yom Kippur, Passover etc.
It is not a sin to use the Gregorian calendar, and for regular everyday life, most Jews do use the Gregorian calendar. It is only regarding the Jewish things such as for holidays, that the Hebrew one has to be used, otherwise you will not be celebrating the holidays on the correct date.
Another important difference between the Gregorian calendar and the Hebrew one, is when the time of the date changes. With the Gregorian calendar, the date changes at 12am, whereas with the Hebrew one the date changes at sundown, and night fall. Therefore, you can use the Gregorian calendar, but when it comes to Jewish functions use the Hebrew one.
By the way it is very easy nowadays to get a hold of a Jewish calendar, in fact most Jewish calendars have both the secular date and the Hebrew one with the correct times and dates for everything.
If you need help getting a hold of one let me know.