The gateway position depends a lot on the type of antenna used.
The Taoglass antenna has a 5 dBi gain and there are other antennas up to 9 dBi gain.
Now we need to ask how these antennas work. A normal 1/4 wave antenna has a donut pattern:
The antenna sits in the middle of the donut so it radiates (and picks up waves) below and above the antenna.
For the Taoglass, and other antennas with amplification, this donut is flattened.
Back to gateway height.
If the gateway is placed high and the antenna is a +9dBi antenna, there is no reception on ground level near to the antenna.
So in the end it all depends what your goal is and it also depends on the environment. In the middle of an open fiels, 5-10 meters can be high enough. As strange as it sound, this is also the perfect height in an area with only high rising buildings. Placing the antenna on top of a high building will reach the area around that building but most likely not the building itself.
Line of sight is not absolutely necessary but it will affect the distance. My office in Eindhoven is located around 3 km from a TTN gateway and 4 km from a KPN gateway. In both directions there are high buildings that block the direct view but reception is still good (RSSI of -114dB for the TTN gateway and -96dB for KPN)