Welcome to the Bunnings Workshop community @RKM. It's fantastic to have you join us and many thanks for your question.
You might like to check with the manufacturer of the roof system as to what it is capable of suspending weight wise. They should be able to supply you with information as to whether cladding with any material is possible.
If you had a builder inspect the roof they might be able to tell you if fixing plasterboard to it was an option. Obviously, you would need to check whether the roof is structurally suitable for that type of weight as it would be significantly heavier than plywood. I would advise checking with a builder, structural engineer or the manufacturer first before commencing work.
Please let me know if you need further assistance or had any questions.