Hi, I seemed to have misplaced my Things Gateway antenna in a move recently and I would liek to replace the antenna but I suspect I can’t just go on amazon and order the first one I see, so what do I need to know in order to get this up and running again? I have a US gateway version.
What does this mean exactly, do you have a picture?
It is almost identical to a WiFi antenna, even though WiFi is at 2.4 GHz to 5 GHz. Not sure if you really need anything else special.
Wrong, US version of GW is running at aro 915 Mhz (905-925ish) and even the small difference from an 868Mhz version to 915 can significantly impact performance & coverage. You need a VERY wide band width aerial if you start at 2.4Ghz! 
A no no. Aerial needs to be tuned accordingly and with VSWR at nominal freq ideally <1.5. Look for one with aro 30-50Mhz usable bandwidth at target freq. (That then allows for slight environment driven de-tuning due to any metals/conductive surfaces in close proximity).
A search on ebay, amazon or aliexpress should yield plenty of options but a good place to start and way to get view of others is to look at the BIG and SMALL Ant thread:-
@MathanCompBioakes you might also want to look at the Bargain Basements Threads (1-4!) to see what deals other have found for good aerials
Note also polarity & ‘sex’ of ant when choosing fit for your GW (SMA/RP-SMA, Male/Female…)
The TTN gateway comes with a single aerial, but is suitable for both European 868 and Australian 915 frequencies. Or is the aerial matched to the LoRa board when you request a regional configuration?
I have bought a 1.5 M fibreglass +14 dBi gain external antenna and the manufacturer states 868-928 MHz, so does cover the full range, or are they being optimistic?
The +14dBi sounds optimistic on its own, but wideband as well ?