Well, I looked up my bill. We live in a very small condo, just me and wifey. Gas WH and gas stove. She does all the laundry in cold, and we cook largely with the convection nuker. Occasionally fry an egg or a pancake on the stove. No gas heat. Electric dryer. Our bill for January is 15 therms, or about 1/2 a therm per day.
I could see that a gas dryer would add a lot for us, because she washes a lot! More us of the oven would add some. More than 2 people would be more hot water. Is your goal to be at 1 therm or what is the question exactly?
Two people couldn't possible generate enough laundry to make the dryer a significant part of the gas bill. It's still going to be at least 75% hot water use.
And 15 therms/month would be about right for a 2 person household with tank HW heater with primarily showering/bathing use. A typical standing pilot ignition burns through ~0.20-0.25 therms day or ~7therms/month. Some of that pilot heat ends up in the tank, but most is lost as standby. The total standby loss of a gas-fired tank is going to be somewhat more than that for standing pilot units.
But for any tank units you can estimate the fuel used to support the standby loss this way: A gas fired tank burns at ~80% efficiency. The DOE EF test assumes a use of 64.3 gallons (536lbs) of water/day with a temperature rise of 77F, so the total BTUs that went into heating the water is (536 x 77=)~41,275. With an 80% burner takes (41275/0.8 =) 51,594 in source-fuel BTUs to generate that temperature rise. But if the unit tests with an EF of 0.60, it means it actually burned (41275/0.6=) 68,792BTUs of source fuel. The difference (68792- 51594) is 17,198 BTU/day, and x 30 days is 515,940 BTU (or about 5 therms/month.)
Mind you, the EF test assumes an ambient room temp of 67.5F, so if the tank is in a room dramatically warmer or colder than that the standby losses will vary.
So for two people 8-10 therms/month above standby use would be a modest bathing budget.
Most gas dryers aren't going to use more than ~80 therms/year (~7 therms/month) even for a family of 4, or about the same order of magnitude of the standby loss on a tank heater. For a two person household it's about half that- not exactly "in the noise" of your hot water use, but not a big adder.