I have a lvl 38 bronzong and its speed is 33. would you recommend teaching him gyroball? and if anyone knows, around what area would the average base power then be for my bronzong?

that depends on your foe's speed stat. From what I understand, the power gets larger with the faster opponent. (ex: it would do more to Deyoxs speed forme than to Golem.

It gets more powerful if you're slow and the opponent is fast. So I'd reccomend finding the IV of its Speed, and if it's low, do it. And what's its nature?

Once you have the IV, you can get an idea. Low = Good, high = bad. But because you'll never be sure of the opponents IVs, EVs or anything else like that, just hope for the best.