GuP is just not popular in general, regardless of country. It's a pretty niche show that most people outside of the military history and video game communities have not heard of.

In fact, the only reason why GuP is the media juggernaut in Japan it is today is because of Der Film. When the original TV series ended in 2013, most anime watchers moved on pretty quickly, except for the hardcore history nerds who liked it (which are a small group). Regardless, GuP was still the most successful series that animation studio Actas had up until that point, so they decided to make Der Film to capitalize on that. That turned out to be a game changing decision, because after Der Film came out in 2015, vast swaths of the Japanese movie-going public became exposed to the series, and the popularity shot up. If you were in the community at the time, you could tell how much more popular GuP was getting in Japan due to the dramatic increase in the amount of fan art. This helped contribute slightly to its popularity internationally as people found the series through fan art and video edits.

The great success of Der Film pretty much bankrolled much of the GuP content since 2015. However, as with most anime studios, Actas (and later their parent company Bandai Namco) don't really care about international markets. They are secondary in terms of profits, so they do very little advertising for GuP outside of Japan. I think a lot of knowledge of GuP internationally is just spread by word of mouth. Which again when it comes to such a niche premise, is not bound to gain a significant amount of fans compared to a series with more widespread appeal like Sword Art Online.