[Occasionally6]

A GST is regressive because different proportions of high and low incomes are spent. An individual on, say, $100K/yr has significant discretion over how much to spend. An individual on, say, $25K/yr is likely to spend all of that just to survive.

If we arbitrarily set the GST at 20% with no other taxes:

Assuming 100% spending, the $25K/yr individual would pay $5K/yr in tax.

With an income of $100K/yr an individual might spend, and so pay tax on, $50k/yr (twice as much as the $25K/yr household) and then pay $10K/yr tax.

If those same taxes were assessed as income tax the $25K/yr individual would pay at a rate of 20% and the $100K/yr individual at 10%.

A flat rate of income tax is a bit fairer but is still regressive in effect because the unspent income is available for investment. It still tends to widen the gap between rich and poor and trap people in poverty.