The standard PSD I have is being displayed incorrectly in GIMP. Colours are missing etc...
I have access to PhotoShop. Which method should I use to save the file under PhotoShop to maximise compatibility with GIMP?
Depending on what all your PSD has, you may be able to get away with
To maximize compatibility it's Edit > Preferences > File Handling. It will differ slightly depending on your version. Here's a screenshot: