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…)