If you wish to try the reverse sear method on the BBQ, simply light 1 burner on 1 side of the grill, leaving the other 2 or 3 burners off, and test adjust with a closed lid until the grill consistently maintains a temp of say 250 F.
On my BBQ, I turn the single lit burner to approx. medium - hallway between low & high, to quickly reach & maintain a consistent temp of 250 F.
Then place your steak(s) on the opposite side of the BBQ (over an off burner- farthest away from the lit burner).
When you've estimated the steak(s) are about halfway to your desired temp (of e.g. 120 F), turn them over.
Remove the steaks to rest (covered with foil) when the desired temp is reached,
and light all your burners to high, and reverse sear your steak(s) when the BBQ temp reaches over 500 F. Works great.
P.S. And as a tip for an added bonus, BTW, here's killing 2 birds with one stone this way:
If you decide to try the reverse sear approach on your BBQ grill, and you also like the added flavor provided through smoking the meat, I suggest that when you light the single burner on one side of the BBQ to heat up (or on the back burner if your grill is configured that way), just immediately place 2 or 3 'Bradley' brand flavour bisquettes directly on the grate over the LIT burner.
By the time you have heated up the grill and stabilized it at your desired low & slow cooking temp, and you're ready to add the steak(s) to the unlit side of grill, the bisquettes should have started to smolder, and begun dishing out some smoke to flavor your cooking steak(s) during the 30 minutes or so time it takes to bring them to your intended temp.
During this time you will likely have turned the bisquettes over once to blacken them on the other side.
When the steak(s) are finished to your liking, discard the spent & blackened bisquettes, and remove the steak(s) to rest while you light all the grill burners to high setting for heating to more than 500 F in order to perform the sear.