2 bedrooms in my house that are on a gable wall are constantly covered in black mould and is damp, is there anything i can do to stop this?

Putting gloss on the walls wont stop the damp, its usually caused by not enough ventilation in the rooms. You need to check you have trickle vents in the windows, if not then retro fit or add some air bricks. The problem could be worse if you have solid walls with no cavity.

Is your propert built with a cavity and if so have you had cavity wall insulation fitted ? because I have come across problems like this last year when gable end walls which had been fitted with cavity insulation started to become damp inside and then mould had started to grow regards Gary Griffiths G R Griffiths Builders

It sounds to me as though you have a condensation problem. You need to allow more ventilation in to the room. The moisture you breath out as you sleep is condensing on to the cold walls. You may have to create a warm wall ie: use insulated plasterboard on existing walls or batten then insulate the existing walls. Note: It is hard to advise with precision without seeing the job. Hope this helps. Regards J Chappell Joinery and Renovation.