We use larger pipe to hang our gates with flat hinges that bolt or weld.
We hammer the hinges so they fit around whatever circumference our pipe may be.
Many here use those poles and if set deep enough, they work great, there is no sag to them, even with heavy gates.
They buy the gates with flat hinges they then use big lag screws to hang them.
If you have the pin hinges that you need to have a hole to stick thru, make your hole all the way thru with a drill extender and the right sized bit.
Hardware stores sell all threaded rods of different sizes.
Buy one to extend the pin hinge and weld whatever you need to reach thru the diameter of that extra long post.
Most anyone in any shop, even a garage, can weld that for you.