Aggregate Willmott and Matsuura Temperature Climate Data on Country Level

I’m looking for monthly mean temperature data on the country-level since 1900, or at least 1950, with temperature weighted according to population density.

The common database for the spatial temperature is by Willmott & Matsuura

The population data typically comes from Gridded Population of the World

I downloaded the Willmott & Matsuura temperature data, which is a single gz-file.
How can I aggregate the Willmott-Matssura temperature data at the country-level using R?

Is there an open code for this? Or has someone else maybe done it
(as it is a commonly used dataset in the literature)?