That's probably not easy; see Single-channel Gateway with RN2483? (And please continue there for specifics about that setup.)
As you still cannot test if you only have a gateway without a node, why not keep the Libelium as a node, and build a simple/limited gateway? Like an ESP8266 with an RFM9x? That topic shows you how to create a single channel gateway with a Raspberry Pi as well.
You would need to change your Libelium node to only use a single channel and a fixed "spreading factor" (SF or data rate) though: "normal" nodes change channel (frequency) for every packet they send, but a single channel gateway can only receive on, well, a single channel.