An eight foot wide double gate will be very hard to stabilize without an overhead connection between the posts. Part of the equation is how stable is the soil around the concrete.

I would use 6x6 posts and set them at least 3 feet deep. If your frostline is lower than that you will have to go deeper.

I would make a hole that is maybe 18 inches near the ground surface but gets bigger towards the bottom of the hole. Kind of an upside down mushroom. The bottom part should be about 24 inches x 24 inches. Make the bottom flat and not round, flat will resist more.

This will be a lot of concrete but the only way to have any chance of stopping the gate from sagging with time.

I also like the small wheel idea to take off some of the weight.

If the gate is part of a fence that continues in line, the rest of the fence will help keep the posts straight some too.

If at least one side can be attached to the side of a building you have all the strength you will need on that side.

A diagonal brace that starts low on the bearing post side and angles up towards the upper outside of the edge will not allow the gate itself to sag.

If you figure the weight of an 8 foot gate, it will all hang from the top of the post pulling it sideways. The higher the fence the more the pressure is multiplied due to the leverage effect.
Not an easy project and if you want it to last expect to put in a lot of concrete on those posts.

54 3100 with 235
62 flatbed dump C60 with 261