Most GPSr's [Garmin, de Lorne, TomTom, etc] have the Internationally accepted default set and that is WGS84. If co-ordinates are given in QNG [Qatar National Grid] and your GPSr is set to the default - which is normally WGS84 - then you will find them to be approximately 160 - 200 metres out. QNG is really only used for official survey documentation and other geodetic documentation.

Qatar is one of the very few countries world wide that has its own National Grid. The reason therefor eludes me as the other countries that have such a grid are Russia [or the former USSR area], Canada and Australia. These are huge expanses of land that Qatar certainly isn't, but I am sure that somebody had a good reason to develop a home grown National Grid.

"To be or not to be? That is the question"