The most important thing is the location. LoRa works best with direct line of sight. The higher you can place it, the better.

Of course, a higher gain antenna will provide an advantage. It will narrow the radiation beam, so it shines more to the sides instead of wasting power by covering the areas above and below.

In an area without any obstacles it will extend your range. In an urban area it can overcome some obstacles by better reflecting the signal off of certain surfaces. Or so I've been told, I'm not a physicist :)

In our mesh, the most popular antenna for stationary repeaters is Mikrotik Omni, 6.5 dBi. You can use any reputable vendor in your area, but avoid the cheap stuff from aliexpress. They often lie about the gain, and can be a hit or miss.

Be careful about the antenna connectors, though! Most of the ready made repeaters come with RP-SMA connectors. The normal SMA, or even the N-type connectors are certainly more popular with the traditional radio crowd. There are adapters, or ready made cables with appropriate ends available.

Then there's the interference. You can have the best location and high gain antenna, but if there's a mobile cell tower broadcasting on a close enough frequency, you're going to have a bad time. This might be highly dependent on the region and the chosen lora band. (433, 868 or 915 Mhz). If you suspect this might be a problem, the RF filter can be screwed in, in front of the antenna.

Hope I didn't scare you. Best of luck!