Is GISTS a Wordle Word?

Yes. GISTS is a valid Wordle word.

It is not on the Wordle answer list, so it is unlikely to be a daily answer.

Plurals ending in S are almost never chosen as Wordle answers.

What does GISTS mean?

The most essential part; the main idea or substance (of a longer or more complicated matter); the crux of a matter; the pith.
The essential ground for action in a suit, without which there is no cause of action.
Resting place (especially of animals), lodging.

Definition from Wiktionary, available under the CC BY-SA license.

Starter word rating: WeakIn the top 82% of all valid Wordle words for testing common letters.
Vowels: I
Unique letters: 4 of 5Repeats S
Common letters: I, T

Hard-mode trap: 10 valid words fit the pattern _ISTS. If you reach it, you may need several guesses to find the last letter.

Letter ratings use how often each letter appears in Wordle answers.

Tap its tiles to the colours Wordle showed, and the Wordle Solver lists every word that still fits, with the best next guess.