Because there is only one place to use  this two variable in our engine:  
`CCASSERT( _maxGID >= _tileSet->_firstGid && _minGID >= _tileSet->_firstGid, "TMX: Only 1 tileset per layer is supported"); `
and because of the gid some times do AND operation to some mask value, in that case, this two variable can't predict anything.
and I changed the gid to int, do AND operation always generate a negative number, that cause _minGID and _maxGID are wrong number, and may triger this assert sometimes.