If my comment is insufficiently detailed please remove it, in particular I have only done any real research into calculating inflation rates from the American Civil War but I suspect that many of the problems may be similar. Calculating historical GDP is a deeply difficult problem. One of the biggest problem is the relative prices of goods have changed drastically i.e. the ratio of the cost of foodstuffs and dyes has changed dramatically over time. On a similar note, according to World Bank data agriculture comprised only 3% of the global economy in 2015. Admittedly for some poor states like the DRC, estimates put their percentage near ~40%, however this is still much lower than ~70%. In highly developed nations it contributed even less. By contrast depending on time period and historian you get estimates like ~70% for the proportion of the Roman economy that was agrarian. This can also make GDP numbers harder to interpret. Finally the data we have from such time periods is often rather limited and if we try to combine data from multiple time periods then we have to make estimates of inflation and changes in prices within a basket of goods both of which introduces more error.

Overall, such GDP estimates can be historically interesting but there are a myriad variety of ways to calculate it, each of which can be useful for different questions and each of which can result in wildly different numbers.

In addition to u/adirocket's answer, I made a past comment about how historical GDP estimates are calculated that you might find useful.

In terms of GDP per capita comparisons, they are typically done in US$ of a particular year. So a figure calculated per 1990 US$ will be lower than the same figure but in 2010 US$ (like the same length in inches versus centimetres). So that's something to look out for.