Gondor has arguably more parallels to the Byzantine Empire than to a Western feudal society, so would have been a tax policy, though I suspect that more of the governments wealth would have come through collecting rent on agricultural land owned by the nobility.

If we continue the parallels to the Byzantine Empire more than the text can take us, much more important to the merchants than taxes would likely have been the government's involvement in the skilled trades (i.e. guilds) and setting of interest rates.