There was some testing on different radios here: Radio Testing.
As for why it doesn't work as well in an open field. I am with the others that suggest interference. Are there any cell tower other antennas that might have transmitters nearby (or not so nearby if directional)? If they are transmitting in the 2.4 GHz range, it could effect it the radio range. The band used by the Flysky GT5 (and probably most radios) is used by multiple things https://fccid.io/frequency-explorer.php?lower=2408&upper=2475. This also includes WIFI. https://en.wikipedia.org/wiki/List_...provides several,into a multitude of channels.
As for why it doesn't work as well in an open field. I am with the others that suggest interference. Are there any cell tower other antennas that might have transmitters nearby (or not so nearby if directional)? If they are transmitting in the 2.4 GHz range, it could effect it the radio range. The band used by the Flysky GT5 (and probably most radios) is used by multiple things https://fccid.io/frequency-explorer.php?lower=2408&upper=2475. This also includes WIFI. https://en.wikipedia.org/wiki/List_...provides several,into a multitude of channels.