TG use several concourses so it's not easy to predict. But, as a guide you could look at the same flight on previous days here....https://suvarnabhumi.airportthai.co.th/flight When you find your flight click on it and it will tell you the gate used.
As for the lounge I have used occasionally but there is no need to book, just walk in and pay.
+1 for Nam's for the taxi. Used them last weekend, perfect service as always.