Hi @tripleV,
Welcome to the Bunnings Workshop community, it is fabulous to have you with us.
Assuming the boards are still relatively solid, then you should be fine to just replaster the corner. If the damage is more significant, you may need to replace the board, however, it doesn't look too bad from your photo.
You'll have to grab a 3-inch plasterboard joint knife, internal corner tool, a mud pan, some paper jointing tape and multi-purpose joint compound.
Check out How to set Gyprock Plasterboard Joints. At 3:10, you can see the internal corner being plastered. This will give you a visual example that you can follow. This Jointing Guide from CSR will show the steps when using multi-purpose joint compound.
Once it has been plastered, you'll want to give everything a good sand with a sanding sponge before painting to finish.
This will give you a nice, clean repair that will look just like new.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
