Hello @NT1,
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about the gap between your wall and roof.
I suggest having a good look at this discussion - How to plaster over new Gyprock plasterboard by @siva26.
The general steps will be to apply plaster filler compound followed by plasterboard tape then filled over with filler compound again.
In regards to the large holes, these must be covered with Pfeiffer 15cm Plaster Repair Wall Patch. If you've not done large scale plastering before, I strongly recommend engaging the services of a professional plasterer for a quick repair.
Here is a handy step-by-step guide: How to stop up villaboard.
If you have any other questions we can help with, please let us know.
Eric
