I will recommend you purchase Flightaware 26 inch 1090 Mhz antenna from PiHut.
https://thepihut.com/products/flightaware-1090mhz-ads-b-antenna-66cm-26in
If you want a low cost antenna, then the one you have linked from “PiHut”, a reputeable supplier, so most likely this mag mount antenna will have gain as advertised.
I have purchased 3 similar antennas from Chinese sellers on ebay. Each was similar to the PiHut one but with different dimensions. Two were advertised as 5dBi and one as 3 dBi. One performed same as the straight whipped mag mount of genric dvbt,and two performed worst than the mag mount whip of generic dvbt.
If you want a cheap solution, go for a DIY Spider made of Coax only. You will need an adapter (or better a pigtail for flexibility) with F-female and SMA-male connectors at opposite ends.
QUICK SPIDER - No Soldering, No Connector