when fixing the side plasterboard, allow it to sit down below the steel flange by 18mm. then dab some board within this slot with full fill dry wall adhesive.
Better still, score the board on one side so that it wraps around the beam.
ie You have a strip with two folds in it so that it can fold into a u shape the same dimensions as the beam. You tack one side aas normal, you can put a bit of adhesive or gripfill on the beam , wrap the boiard around it and tack the other side as normal.
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