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Gateways are between the end device and the network server.
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On the end-device and on the server side I've represented 
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the message as layers.
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When we use this representation, we consider that each layer add a service to the one below.
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So let's take the example of our temperature sensor with one byte user payload.
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This temperature belongs to the application layer and is encrypted with a key called AppSKey.
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Then, the layer below will add the LoRaWAN header. In the Lorawan header,
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the main service is the authentication process and this is done thanks to another key call network session key.
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And finally, this lorawan message will be modulated in Lora and a message sent as a radio frequency
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packet.
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I remind you that the transmitted message is not sent towards a specific gateway. All gateways in the
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coverage area will receive this packet and demodulate it.
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On this diagram I've drawn one gateway and we can see that its job is simply to demodulate the message.
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Once it has recovered the PHY payload,
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so the exact content of the lora frame, then it sends it on an IP network (over internet).
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A gateway doesn't analyze the lora frame, so it doesn't authenticate the packet,
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neither decrypts it. 
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Anyway, they don't have the network session keys and the application session keys, so there is no way they
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can manage it.
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So we need to consider the gateway as a simple Lora entity with emission and reception capabilities. They
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receive the radio frequency message on one side and transmitted to Internet on the other side.
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The IP network is usually called the backhaul, and it's often Ethernet, Wi-Fi or 4G connection.
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And finally on the right, it would be up to the network server to authentic the packet.
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Then the final user payload will end up to the application, server ready to be decrypted.
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If everything goes right, the temperature value should obviously be the one sent by the end device.