Sinking steel gate posts into the ground - too short

I think we need picture? Are these cast iron posts or steel.

Another option is to do what you can (set in concrete will probably do the job anyway) and then tie the gate posts to solid brick or stonework pier to the side of them.

OP you might want to consider getting those posts galvanized?

There are loads of engineering applications where steel posts are bolted to concrete, with none of the steel going into the ground. The issue isn't how much steel is in the ground, it's how stable the concrete foundation is.

When I read your post I thought you had some fancy antique cast iron gate posts. What you have is two short lengths of steel pipe. My advise is don't use them, instead build two stone or brick pillars that will last and look the part. The steel pipes will rust & rot and painting them is pointless. But if you decide to use them as pillars you will need to weld on top brackets to hold the gate, so it's best to get the gates made first and weld on bracket to suit. The height of the gate will decide how deep the pipes can go - as the are short you will have to weld on at least 900mm, also weld on, say, 4'' bits of angle iron on this extension, it will help stabilize the posts in the concrete. If you're fitting a steel gate (which will have to be galvanized) get the pillars galvanized as well.