Gitzo’s “Series” designation is generally a size and load-class indicator, not a quality tier. In practice, the numbers run from very small/light models up to larger/heavier ones, with lower series intended for lighter, more compact setups and higher series for bulkier gear.

So for tripod heads, a Series 1 head is typically smaller/lighter than a Series 2 or Series 3 head. Likewise, your Series 0 tripod is on the lighter, more compact end of the range. That makes the series useful mainly for matching the physical size and carrying capacity of the head to the tripod and camera kit.

Based on the available information, the series numbers do not indicate that one line is “better quality” than another; they mainly indicate size/weight class. For exact decoding of specific model numbers and the differences between head families, Gitzo’s official documentation is the best source.