Choosing a good antenna is arguably the most difficult task in setting up a gateway. Not because antennas are hard to find, but because reliable information about them is hard to find. But I for one would place all the antennas you link to in the ‘avoid’ category, because these are multi mode wideband antennas, not especially suited for 868MHz. There have been some tests on this forum, and most of these antennas even attenuate the signal in stead of increase the reception.
In selecting an antenna, next to the frequency range you should at least have information about the gain, the radiation pattern and the VSWR. But even then, if this information is not coming from a reputable manufacturer it is to be taken with a grain of salt.
If you want a cheap solution that works pretty well, you can easily make your own antenna. For cheap antennas, some people here (including myself) have ‘good’ experiences with this and this antenna. ‘Good’ as in: it works at least as well as the DIY ground plane antenna.
Like said before, even 2km range will be difficult if the antenna is low and in the vicinity of obstacles higher than itself. To guarantee good reception, you should be able to draw a straight line between the place the node is positioned and the antenna. But you will have to experiment. Start with the DIY ground plane, this antenna has no gain and therefore a wide radiation pattern (no directivity), which you will need in your case. This means that this antenna will be able to pick up signals in almost all vertical directions. Antennas with gain, like the ones you link to or the Aliexpress ones, have a more horizontal radiation pattern and are less sensitive above and below the antenna.
The reason I recommend placing the gateway outside as well is to avoid cable loss. Cheap coaxial cable, like the ones attached to the antennas you link to, can have a loss of up to 1dB/m. If you have a cable like this with a length of 6m, you won’t have much signal left at the other end. The closer you can place the gateway to the antenna, the better. Or buy good cable (e.g. Aircell).