Affix gutter to pole

Epoxy would be a better choice but I think I would just drill a hole and put a screw in there or a pop rivet. Another choice might be a hose clamp.

Long or multiple Zip Ties ???

My first thought was hose clamp. There are also these wrap locks: They come in lengths up to 48". One can hook two or more together if that isn't enough. One can always cut these off to fit also. or these The largest I've seen will fit a 10" diameter pipe. One has to be careful to get the right size though. They're heavier so it would be harder to get them to bend for your situation.

I assume this is a downspout, not an open gutter?

Use the metal straps sold for affixing the downspout to the structure and screw them to the pole using #8 self drilling tapping, stainless, sheetmetal screws.

OR

You could use UV resistant, high quality zip ties / tiewraps.

OR

You could use stainless steel bailing wire wrapped around the downspout and pole and then twisted behind the pole.

Personally, I would probably give the tiewraps a shot, using a color that blends in with the downspout. Quick and easy.

I have doubts about the long term use of tie wraps. They would probably age out rather quickly. The metal downspout straps are probably the best solution.

Bill

Or a tie temporarily with a dab of construction adhesive or silicone caulk behind, remove the tie when it cures.

Maybe, but when I use the high quality, UV resistant wraps, such as the ones made by Panduit, I find they last outside for many, many years.

He could also use stainless steel tie-wraps.

Assuming the only load to hold is against the wind, construction adhesive should do it. Clean it well, apply, tape it in place until cured.

Bill Gill posted for all of us...

One can also use the outdoor rater weather resistant ties-usually black.