Don't worry so much about gain, for your sanity. When buying antennas, it's usually just a number they made up. Gain is a measure of the directivity of the antenna's radiation pattern, so a 10DBi gain antenna would be very different antenna like a Yagi antenna that has to be pointed directly at a another radio to work.

The advice about 1/4 wave antennas is good. Antennas need to be a fraction of the wavelength of whatever frequency they operate on. 1/4 wave is sort of the standard antenna that always works well. I prefer 1/2 waves myself, the radiation pattern is not all that different but a little better in the horizontal and they work better without a ground plane.

1/4 wave for 868MHz is around 3in or 7.6cm and 1/2 wave is around 6in or 15.2cm. So, basically, any antenna around 3 or 6in or 7.6 and 15.2cm should give you the best performance in antennas of this style and size. The giznot is 17cm, which I assume is a 1/2 wave plus the base.

When it comes to Meshtastic, I'd say the longer antenna will be better, but if you want to put your device in your pocket or something, a shorter one will be fine too. The good thing is, the antennas are pretty cheap. If you get the Giznot you can also get a 3 inch antenna for like $3 and use them both.