It doesn’t boost the signal the way a Wi-Fi booster does. It’s just that in a mesh network like the zigbee home automation that smartthings uses, one device can pass along a message meant for another device.
Think of it like the old pony express. The first horse rides as far as it can, say 20 miles. Then it stops at a way station and the rider gives the mail pouch to the next rider, who takes a different horse And they ride the next 20 miles to the second waystation. Where again the mail pouch is given to a new rider Who goes the next 20 miles.
It’s the same way with Zigbee messages. If each device can send a message about 40 feet and you need to cover a distance of 55 feet, if you put a device that is capable of repeating about 30 feet from the hub then the message will get sent to the first device (30 feet) who can then transmit it on to the second device which is another 25 feet away.
Most mains powered zigbee devices will repeat except for Sengled lightbulbs. Most battery powered devices will not.
So in your case, position the GE pocket socket so that it can successfully transmit to both the hub and to the garage sensor. Then it will pass messages between them.
So you aren’t boosting the signal, you’re just adding another rider to the process. You’ll likely place it about 3 m in from the edge of the range from the hub. That way it will have a strong signal to the hub but it will be able to carry-on for another 20 to 30 feet past your previous range limit.
For more information, see the following FAQ. Start with post 11, read that, then go up to the top of the thread and read the whole thing.