I have a light gate thar collapsed because the post was rotten and it needed to be replaced. I want to replace the post but to avoid setting it in concrete so I am thinking of using a post spike and set it in concrete but I am not sure if the grip at the base is strong enough..
I think this is not recommended
The concrete will contract and it will dilate and the shape of the spike will facilitate the pushing up of the spike in winter when the concrete contracts. This will ruin the grip that the concrete needs to have on the metal in order to have a rigid assembly there
The situation is different when you use any of the above anchors but I do not thin that those are appropriate for the situation when the post holds a gate When the gate swings it moves the load in a lateral plan and the above anchors are mostly designed to support vertical loads
You could, as an alternative, be more successful digging a hole and setting a longer (taller) post in a couple feet of packed stone. The stone will hold the post just as securely as concrete and allow the post to be replaced more easily in the future.
Some people feel that setting the post in crushed stone also prolongs the life of the wooden post because it allows rainwater to drain away from the post better than a situation which allows water to collect between the post and a concrete base (or in this case, a spike collar) and remain in contact with the wood for a longer period of time.