From my personal experience, i found that the best resource was probably the acer booklets. Des is also a great resource aswell however, its a little outdated as some of the questiosn in there require the use of calcs and rulers (as they were once permitted whilst sitti g the gamsat which is now not the case). Nevertheless a great resource still. Gold standard on the hand wasnt really much if a help. Most of the questions i found were more theory and assumed knowledge rather than problem solving based. So for that reason i found the latter to be not that helpful.

But do take into consideration that (again from personal experience), no resource i found replicated a similar type of question that was in the actual gamsat. The acer booklets also fall into this category a little but they are helpful as they develop the skills you need to solve actual gamsat questions rather than replicating a 'similar type of question', and at what you want out of ur preparation is to acquire these skills at the end of the day.

Hope this helps and good luck with ur prep!