Noah,
It really depends on the construction of the pillar. If it is constructed of blocks on the flat then bolting to the pillar is sufficient providing long enough fixings are used.
If it is a stone pier and back filled with concrete then you can still bolt using chemical anchors providing the threaded bar is anchored into the concrete behing the stone.
If it is a big solid wooden gate or very heavy iron gate that is going to be subject to wind loading, then fixing onto a steel support running up the middle is advisable.
Your gates are not that big so bolting should be fine.
Paul24