Sure, the tuk tuk drivers get a commission for bringing guests: two dollars, which is not added to the guests' bill. It's a fairly common practise in which other hotels in the area participate. Having said that, GBT 3 guesthouse has a great location, right near the beach and shops, and in the same street as all the diving operators. Sihanoukville being a spread-out city, this is definitely a selling point.
We stayed here twice. Room prices went up in the meantime, due to high season setting in, but remained competitive. The rooms were spacious and clean, minor problems were fixed promptly, and since all the staff communicates via handheld transceivers, orders (to the restaurant kitchen, to the cleaning staff) were also carried out promptly.
The staff was business-like but courteous. All in all, an efficient operation and a good jumping-off point for dive trips or looking around for a place to stay in a more remote and quiet part of the area.