can you normalize by the area as you would generally do for a choropleth?

You just have the choice between 2 operators : mean or sum. See grid.js on my github repo. See also my notebook regulargrid. Suggestions are welcome.

Sorry, I guess I was looking for the "sum" operator, but I didn't understand that it would normalize by area. When you show population, it seems much better with the "sum" op.

Complicated. Because as it looks like 3d, what should be considered? The volume or the height? I don't have a definitive answer on that...

For me the rule of thumb for choropleths is the following: if you split a region into separate entities, the map should not change (at least not drastically). In this case (population or gdp), with the "mean" operator Europe —being split between 25 countries— disappears completely, although its total population is 5x Russia's. If I understand how this map is computed, I would say that if I use it to display a rate (e.g. mortality, or gdp per capita), then "mean" is fine, but if I want to display an absolute quantity such as population or gdp, then I should use "sum". Is this correct?

Yes. I think that what I propose is more clear in this notebook. https://observablehq.com/@neocartocnrs/bertin-js-regular-grid