What does GTR mean?

GTR Means "Gross Total Return" - meaning the total return (price return plus dividends) of the constituents without taking taxes into effect.

Obviously that doesn't mean that a fund tracking this index is tax-free, what it means is that the investor would still be responsible for any applicable taxes, and since taxes can be different depending on the investor, they are not reflected in the index.

Gross total return (GTR) indexes reinvest as a lot as possible of an organization’s dividend distributions. The reinvested amount is the same as the total dividend amount distributed to individuals residing in the country of the dividend-paying company1. Gross total return indexes don't, however, include any tax credits.

Net total return (NTR) indexes reinvest dividends after the deduction of withholding taxes, utilizing (for worldwide indexes) a tax rate relevant to non-resident institutional buyers who don't profit from double taxation treaties.