Why Is The Time Zone In Iceland The Same As GMT, Even Though It's On A Different Longitude, Though France On The Same Longitude Has A Different Time Zone?

The official time in any country is determined by the government of that country. This is called Local Civil Time. They may determine their time to be whatever they choose regardless of their longitude or of anything else.  Most countries set their clocks a whole number of hours before or after Coordinated Universal Time,  known as UTC.  (Google it) which in Britain is still known as GMT.  The government of Iceland has chosen UTC  as their official time and that of France is UTC plus one hour.  For a map of the times zones of the world see; aa.usno.navy.mil