If it's the same thing we call Gyprock over here then it also has gypsum in it. Gypsum can be helpful in improving the structure of some clay soils (say for house vegetable and flower gardens), and it wouldn't do any harm. I don't think paint would do any harm UNLESS it was old lead-based paint.
As an old construction worker who has given this issue much thought, here are the objections I can offer. We all buy gypsum to help improve clay soils and make them more porous and improve fertility. So why not use scrap ****rock?
The answer lies in the manufacturing process. Not a thing wrong with the gyp itself, the problem is in the paper backing. As you all know drywall mud is water based and the gyp board gets repeatedly wet in the installation and finishing. To eliminate the problem of mould the paper backing on gyp board is treated with some mighty ugly fungicides. I've let that stuff sit out in the weather for literally years and the paper never seems to degrade. If you have any intent of natural or organic methods keep that stuff away from your fields and gardens.
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