Properly geared there are gasser options from the big three that will effectively tow a 8K# 5er up most any mountain, but none will do it to the degree a diesel will if that is what you truly want.
The newer Hemi 6.4 Ram gasser is a beast and is being used by current forum members to tow heavier 5ers than what you are planning to tow. Mileage reports when not towing is pretty good compared to HD gassers of the past and is not a bad option as a daily driver.
We use a Chevy 3/4 ton gasser with 4.10s to tow our 10K TT toyhauler around the mountain west and it has taken us up every mountain thus far, but I wouldn't and don't use it as a daily driver as it does consume quite a bit of fuel even when not towing. It's a dedicated TV and just sits in the garage until called to duty.
If you want the torque of a diesel than you have to go diesel, I know a few that use them as daily drivers and they seem not to have much problems doing so. Yeah they are a bit more expensive to purchase and maintain but they have better resale and of course more torque if you do a lot of heavy towing.
Ultimately, it's your call. Need to figure out where and how often you plan to actually tow, and if you are planning to go heavier than your 8000# 5er in the future. Diesel would be the better choice if going significantly heavier somewhere down the road, but for a 5er weighing 8K# and just towing 6 times a year or every 2 months, a newer gasser would fit the bill nicely as well.