Surely you agree that if he instead were to hire an extra gardener with the same $30,000, that money would be taxable as the gardener's income? So why should somebody who works for their money pay taxes on it, but not somebody who receives the money as a gift?
But to your point, a gift tax also stops people from “gifting” their gardener and avoiding taxes on both ends.
I stipulated that it is the SAME money. In both cases, it has "already been taxed" by the person spending the money. Yet in one case, the person receiving the money pays taxes, in the other, they do not.
So i would argue they aren’t the same.
In the UK there's inheritance tax to pay over a threshold if you die within 7 years of making the gift, but otherwise there's not a 'gift tax'.
The situation you describe still isn't the same as declaring it as income though - assuming you are working, if it were taxed at your marginal rate of income tax you'd already be over any threshold no matter how small the gift.