Greta is the short form of Margareta — the German, Swedish, Italian, and Hungarian form of Margaret — ultimately from the Greek margaron, meaning "pearl."
The name has appeared in the U.S. top 1000 intermittently for over a century and is currently rising again, helped along by both Garbo's legacy and Thunberg's contemporary fame.
Greta does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Greta reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.