I am planning to use The Things Network for my next project with around 50 LoRa nodes geographically distributed in an area. I am wondering is LoRaWAN free to use - provided we buy LoRaWAN gateway from TTN.
Yes it's free.
The idea is similar to the original Internet model, if you set up a gateway you allow others to use it as well and in return you can use other peoples gateways, that way the overall coverage grows and everybody benefits.
You don't need to buy a gateway from TTN, you can use any number of gateways*, but I think TTN are now asking that you use proper multi channel receivers that support multiple spreading settings and not just basic 2 channel rigs as they don't really add much utility.
* I am currently waiting on this kickstarter to deliver my gateway and a pi hat client board to set up a gateway in my home town.