I made up gates for the side and main entrances of a house I was working on last year. I suggest you choose between pressure-treated softwood and hardwood.
Treating softwood yourself is not the same - even if you brush each part individually before assembly it's not the same as pressure treatment. Wood will rot where it's in contact with the ground or other wood, where water cannot easily escape, endgrain exposed to rain etc; where the wood crosses or joins will tend to stay wet unless you design out as much of this as possible. Through-sawn softwood may tend to warp and twist as you'll be taking it from dry storage and exposing it to the elements.
I used pressure-treated softwood. Found it okay but difficulty to add any kind of stain as what's there already resists that. If I was doing it again, particularly a small gate, I'd probably use hardwood.
My two cents' worth....