What is the best way to compare the price of a GB of internet across countries
Purchasing Power Parity (PPP) is probably the most convenient way of doing such comparisons so you are on right track. As mentioned in the comments there are other ways how to do international price comparisons but PPP is probably one of the few ways that do not require much auxiliary estimation/research as it is already readily avaiable to public.
PPP works in a very similar way to an exchange rate. In fact PPP is sort of an exchange rate built under the assumption that the law of one price holds (Krugman et al (2012). International Economics: Theory and Policy). Consequently PPP is an alternative to market exchange not another factor you use on top of it (so the calculations in question are not appropriate).
For example, if in China 1 GB of data costs $¥10$ and PPP is $4.15 \frac{¥}{\\\$}$, the adjustment would be made as: $¥10/(4.15 \frac{¥}{\\\$})\approx {\\\$}2.41$. Afterwards, the prices can be compared as apples to apples so in this case 1GB of data in China would be more expensive than 1GB of data in US (which according to your data costs ${\\\$}1.37$ ), once adjusted for PPP (although I just made the $¥10$ price up you should use your prices).
Note, even tough it is not always stated PPP is implicitely expressed in $\frac{\text{local currency}}{USD}$ since it is based on comparison of price levels of two countries and by convention US is always in denominator. In fact most datasets will even warn you about this. For example, OECD dataset reports its PPP as:

Purchasing power parities (PPP)Total, National currency units/US dollar,
[emphasis is mine]

You can read more about PPP in Krugman et al. International Economics: Theory and Policy 9th ed. Chapter 16. It is an undergraduate textbook so it contains some worked simple examples.