Welcome to the Bunnings Workshop community @Jarnzy. It's fabulous to have you join us and many thanks for your question.
Firstly, I'd like to apologise for the delay in my reply.
Our helpful members will need some more details on what type and size of gate you are installing. We'll also need to know what the dimensions of the post are.
Ramplugs are appropriate fixings to use in Hebel. The size of the plug and screw used will be determined by the size of the gate and post. If you can provide those additional details I can advise further on the exact products to use.
If it is a large gate then it would be preferential to concrete the post into the ground instead of attaching it to the Hebel.