Also bear in mind that 6dBi gain is effectively a doubling of the distance when compared to an isotropic radiator (if within the squashed polar pattern).
Lets say you’re only getting 300 metres at the moment with a reasonably performing cheap chinese dipole antenna (as above) which has been badly placed indoors.
You might think that your new “6dBi” antenna would double the distance to 600 metres if you put the antenna in the same place.
Nope - Whilst Field strength (E) equates to 7 * Root(erp) / distance, and the square root of 6 db is 3db (that is why it only equates to a doubling of distance rather than x4) the cheap Chinese dipole already has about 2 dBi gain anyway, so when you do the math, there is really a 4dBi gain (6-2), which equates to a “square root” distance gain of 2dB (4/2), which is roughly about a 50% increase (e.g. 300->450 metres ).
However, if you put that new “6dBi” antenna on the roof…Well, all of a sudden it becomes the best performing thing since sliced bread. 
Hence the reason why a lot of people rave about their new outside antenna, but the real reason why it is so good, is because it is better placed.