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I'd replace the post.
The foam might work for a hot minute but I'm not even convinced of that. I'd think the stress from the weight of the gate will cause it to fail rather quickly.
If the hollow is up in an area where the hinges attach you might split the post with the foam expanding as it cures.
This sounds like a bad idea. Just replace the post.
Time to get a new post... This sounds like the equivalent of duct taping your fence together.
Get longer bolts, go all the way through the post, then a metal plate on the other side. The plate is to distribute the pulling pressure. This is what is done with eg. brick walls.
This is a good idea in general; gate hardware usually attaches by going completely through or around to the back side of the structure. Essentially you do not want the structure to hold the gate hardware with tension, you want the structure to hold the hardware with compression.
The thing about the expanding foam is it may force you to get a new post anyway.
When that stuff expands, it can push cracks apart and if you have enough hollow in that center, it may be enough to provide decent compression power to the foam.
On the positive side, the foam will keep water from going in there, which is causing/exacerbating the damage.
Will foam provide support? Marginally. Bolts won't get much added strength, but the entire post may flex a tiny bit less, assuming it doesn't split during the curing period.
Silicone may work better, but I'm betting the quantity needed would surpass the cost of having someone replace it all.
You won't be out much if it doesn't work, but if it does end up splitting the wood, you may want to be prepared to replace it.
Most of the expanding foams are not waterproof or water resistant. You have to paint or caulk over them to prevent them from absorbing the water
If you're in the US, replace it with a western red cedar post. It costs more but lasts much longer than standard SPF 4x4s. Let your kids replace the next one.
Or a 3" thick gauge steel pipe, if you're not concerned with looks. Your great grandkids can replace that.
Foam sounds terrible for holding bolts. If you have to fill it and have a clear hollow access, Iโd consider epoxy and some wood dowels. The dowels would be to bring the cost down because you will probably need more than a gallon of epoxy and that stuff is expensive. This is my idea, please feel free to poop on it.