If you're not going to plaster it yourself, then let the plasterer prep/fill the gaps etc.
but,,,
If "you" are going to attempt to plaster it yourself, them get some bonding coat plaster to fill up the gaps where required, ie, joints/around edges etc. Let the bonding more or less set,( i do it that way), then put tapes over the board joints, and also put tapes all around the wall/ceiling line. You're then ready for plastering.
An extra tip,,
"don't" put the tapes on first and expect the bonding plaster to fill up the gaps through the mesh, it wont work,, fill the gaps up first,, flush off neatly,, then tape. If any of the "bigger gaps" involve p/board edges that have been cut,, exposing the "dry edge/s" of the p/board, it's always worth them damping them down with a small brush, and also brushing on some PVA, "just before" you fill up the gap,, gives much better adhesion,,, so you fill the gap/s with the bonding, while the PVA is wet.