Apple doesn't pay the GST on the sales, that is paid for by the consumers. (Yes I know that is a gross simplification but that is the general idea of GST - it is a consumption tax).
Its a vast oversimplification to say that all corporate taxes are just paid by the customers anyways.
Income taxes are a little different, one business may be more profitable than a competitor, and they may have different profit margin requirements to justify building/selling their products, so the adjustments are going to be different per company. But essentially prices are adjusted fir tax levels within what demand allows.
Imagine income tax is increased from 0% to 50%, any business that was already making marginal profits needs to increase prices or go out of business. Assume Dell makes 4% profit margins on pcs, and Apple 20%, Dell can increase prices 2% to retain its after tax profit levels, Apple needs to increase prices 10%, but may not be able to given its competitor Dell did not.
You seem to be assuming that, if GST was zero instead of 15%, the prices consumers paid for Apple products would be 13% lower than they are now, and that despite these lower prices no additional purchases would occur. Why?
As to what you intended, perhaps. Perhaps not. I'm not an expert in sales of Apple products, you'd need market research to see if a 13% discount would result in an appreciable amount of sales.