On any stuff they don't order online from overseas vendors.
And on any stuff that their companies don't pay for and then claim back as 'legitimate business expenses'
Poor people spend a higher percentage of their money on the basic necessities of life than rich people do. Food, power, water, transport, communications. And richer people are more likely to be able to bypass the GST in various ways - as mentioned above, by getting freebie entertainment or travel etc as work perks, or by spending their money overseas where GST is not applied.
Tax avoidance by the rich using trusts etc is an argument against trusts, not an argument for a GST.