See The BIG and SMALL ANTENNA topic part 1 for a cheap 868MHz center-fed dipole whip antenna, SMA, 19,5cm (adjustable).
I use this antenna for the gateway too.
(RAK831 RPi gatewa. Note: RAK831 uses RP-SMA instead of SMA but I replaced the onboard connector. The same seller also sells the antenna in a RP-SMA version).