I've read that it's good to set gate and fence posts in gravel instead of concrete to help prevent rot and subsequent breakage at ground level. I've just bought a couple of gate posts and now can't find any 'how to' articles

I suppose I dig a hole, but how big? Put a layer of gravel in the bottom? How deep? Then put the post in and fill around it with gravel, compacting it down. What type of gravel/aggregate to use?

Thanks for any pointers.

As SteveZ says its variable, however i would never concrete a timber post in as this inevitably leads to problems such as movement and rot. The other key question is what are you going to hang on them and how heavy / big is it ! What are the current dimensions and materials of your new gate posts length and width ? Are you going to be straining a wire fence against the posts as well as hanging a gate if so is it mild or high tensile ? Is it one long gate or two that join in the middle ? As SteveZ says what are the ground conditions ? Build it right and it will never bother you..... Build it wrong and every time you need to open or close it its going to drive you mad !

I have a similar gap at the side of my house to the boundary but only one gate. The hinge post side, I "strapped " to the house wall to give a good firm fix. If you did that It's possible to have that side out of the ground altogether and perhaps into a metal ground shoe.
For the other side you may be better off with a hardwood post, or for something different maybe concrete with timber cladding to match the other, or steel tube.

If you are on clay the hole you dig may fill with water. Just concrete it in past ground level and haunch around so the water flows away from the post. Min 30" of post in the ground, 36" if gates are heavy or ground is iffy.

Gate posts..... So i have looked at what you have got and can make a few comments (rightly or wrongly) personally i would be using a 200mm x 200mm diameter post if it was treated softwood.
I would never dig a hole less than 900mm deep on the low side and preferably 1000mm. I would use the traditional method of rammed stone to secure the post. This method involves driving in angular rocks around the post until they are wedged incredibly tightly, you start at the bottom and work up. The first and last 200mm are the most important as this is where the strain is, however i always make sure that the whole post is secured tightly from the bottom to the top. I drive the rocks in with a 1.6m crow bar which has a flat rounded end welded on it, big rocks first then drive wedges into the gaps until it all becomes solid then just work on up ! This method allows natural drainage and does not hold water next to the post, it is however tricky to do well if you are not experienced. The posts that you have are just to short for this method as you must get a minimal of 900mm in good ground, if the ground conditions are not good then you need to increase your hole depth and may need to attach a bracing "foot" near the very bace and at the back of the post and another just below the surface on the inside of the post at the top. If you research traditional methods for putting in a "fence strainer post" you should be able to get some diagrams. Just remember your not going to be fitting a "fencing strut" so the solid securing of the post is going to be vital. The problem with concrete is that it never gives a good seal to the wood, as the wood expands and contracts. Water always get down inside and this cannot escape, this leads to early rot especially if you are not using proper pressure treated timber designed for a gate / strainer post....... A large hole filled with concrete and a quality metal foot designed for the specific purpose of a gate post would be a much more expensive and technical way to do it..... But i like the old traditional ways..... Thats just my tuppence worth.

Posted By: adwindrumhttps://www.youtube.com/watch?v=BLx4e_Y0eU8">https://www.youtube.com/watch?v=BLx4e_Y0eU8

looks easy....

"Do not use for structural posts (piles, etc.) or posts under tension (clothesline posts, flagpoles, gate posts, etc.) ..."?

I put in posts the same way DeClock does it. I made this up myself, so glad to hear I'm not crazy. I use some gravel to fill the interstices between stones, otherwise I assume soil will migrate in there eventually.

It is important to have enough depth, as he emphasises, especially for gate posts. The ones that I had for my gates were too short, so it was probably only 0.6m or so underground, which for big heavy gates wasn't enough so they sagged a few mm together after a couple of years, despite chunky 6" posts.

None of the posts I've put in this way got anywhere near rotting, so I don't know how long they'll last. I'm pretty sure it'll be better than concrete, and it's greener to boot. It's a good game bashing stones in whilst keeping the post properly upright. A beautiful assistant is best to help with this.

If you are happy with the aesthetics, I'd always get a cheap length of thick steel (angle, or a very thick, stiff bar), put a peg/old bolt through to stop it slipping down, then concrete that in. It's easier to set a short steel post vertical than a 2M post anyway. Then just bolt the wood to it above ground level.
I normally slap a thick layer of Waxoy on the steel before putting it in. If you chose ~10mm section is about 10 times thicker than an expensive "metpost", and it will outlast you.