The performance of Low Power Wide Area Networks (LPWANs), where LoRa networks are included, has spurred much interest in recent times due to the high interest in connecting more and more devices to the so-called Internet of Things (IoT). The massive number of connected devices associated to its random spatial deployment and random network access introduce new degrees of freedom, demanding for innovative performance evaluation approaches. In this paper we study the interference caused to a LoRa gateway due to the uplink transmissions of multiple coexisting LoRa devices. By admitting an uniform spatial distribution of the devices, we show that the aggregate interference caused to a LoRa gateway follows a non-Gaussian distribution and its power can be approximated by a sum of independent non-identical gamma random variables. Our work adopts a typical LoRaWAN operating scenario where the transmissions of LoRa Class A devices are affected by path-loss, shadowing and Rayleigh fading. Numerical results obtained with the modeling methodology are compared with simulation results, and the validation of the proposed model is discussed for different transmission probabilities.