Gregorian calendar is used by most countries. There are countries that do not use this calendar. Some of the countries that do not use Gregorian calendar are: Afghanistan, Iran, Ethiopia and Nepal.
Afghanistan and Iran use the Solar Hijri Calendar, Ethiopia uses the Ethiopian calendar, Nepal uses Vikram Samvat.
Some countries use Gregorian calendar with other calendars this includes countries like: Bangladesh - use Bangla calendar, India - uses Indian National calendar and Israel - use Hebrew calendar.