How to deal with gaps between plasterboard and adjacent finished wall?

[BenjyK]

Hello. I'm preparing to do my first proper bit of skimming, which will be onto new plasterboard. I've got a situation in a couple of different places where the adjacent wall has already been plastered, but there are gaps in the corner where the plasterboard meets the finished wall. I'm not sure how this kind of situation would normally be dealt with. In one corner the gap is 3-4mm. In another more like 10-15mm.

Is it acceptable to simply fill the gap with a jointing compound and skim over it? Or should I apply paper tape to the corner and then sort of 'tape and joint' over the half of the paper that overlaps the adjacent wall? If that makes sense! Or something else...

Thanks!
Benjy

 

[Wayners]

Better not to have a gap by fittings board tight to wall.
I'd brush in gap some PVA follow guide on can.
Give it an hour and fill with toupret powder filter.
When set fit corner tape. I do this when filler is set so no need to seal. Only need to seal dried filler so easier to do same day.
Next day PVA and skim.

Might be easier to tape It all day before. Then PVA and skim

 

[PotatoSoup]

What's under the gap? Is it a hole or blocks or wood, or something?