Let's break down the relevant sequence of events here, using both Avengers: Endgame itself and the script for reference.
Firstly, in 2014, we see Rhodey with Nebula in the Temple Vault on Morag, having just obtained the Power Stone. With their mission seemingly complete, Rhodey jumps back to 2023 with the Power Stone, and Good Nebula is about to follow, when her memories are suddenly hacked into by Ebony Maw, from Thanos' ship, causing her to collapse to the floor, seemingly helpless.
She remains that way until Maw breaks the connection, and it's at this point that she realises that Thanos knows what the 2023 Avengers are up to.
INT. MORAG, TEMPLE VAULT - DAY (GUARDIANS)
ZZZT. ON THE FLOOR, NEBULA’S EYES SNAP OPEN, HORRIFIED.
NEBULA: He knows...
She then hotfoots it to the escape pod, where she tries to make radio contact with Hawkeye and Black Widow, who are also in 2014, attempting to acquire the Soul Stone on Vormir. She's unable to make contact though, before Thanos' ship arrives overhead and pulls the escape pod up with a tractor beam, with her still inside.
EXT. MORAG - DAY (GUARDIANS)
BOOTS POUND ACROSS THE ROCKS. NEBULA RUNS FLAT OUT, DESPERATE, HEADING FOR THE ESCAPE POD.
INT. ESCAPE POD - DAY (GUARDIANS)
NEBULA rushes to the console and opens A COMMUNICATION LINK.
NEBULA (INTO COM): Barton, Romanoff, come in. We have
a problem. Thanos knows! Thanos-
Suddenly, A SHADOW falls over the Pod. Nebula looks out the
AS A TRACTOR BEAM PULLS THE ENTIRE POD SKYWARD.
The movie then cuts away to a couple of back-to-back scenes with Cap and Tony, the first in NYC in 2012, during the events of the first Avengers movie, and the second at a US Army base in New Jersey in 1970, where the Tesseract was stored.
The movie then cuts back to Thanos' ship in 2014, where we see Good Nebula getting roughed up by Bad Nebula, who promptly rips Good Nebula's time-space GPS from her hand.
INT. SANCTUARY-2, PRISON CELL - DAY
WHACK! NEBULA GOES TUMBLING ACROSS THE FLOOR. She gets to
her knees, looking up at...BAD NEBULA, GLARING DOWN AT HER.
BAD NEBULA: You’re weak.
NEBULA : I’m you-
BAD NEBULA: Shut up!
BAD NEBULA PUNCHES GOOD NEBULA DOWN. GOOD NEBULA GROANS AS
BAD NEBULA RIPS OFF HER TIME-SPACE GPS.
So from that point forward, Good Nebula simply didn't have the option to jump back to 2023. She did have a window of opportunity to use it before that, from the point that she woke up on the floor in the Temple Vault, at least up until the point that she was beamed up to Thanos' ship.
But what would she have achieved by going straight back to 2023, without letting the Avengers already in 2014 know that Thanos was on to them first...? The Avengers in 2023 didn't have any Pym Particles left, so it's not like they could send anyone else back to help. They were only in a position to affect events taking place from 2023 onwards.
The only Avengers that could prevent Thanos from getting hold of the Infinity Stones still in 2014 were the one already in 2014 themselves, i.e. Hawkeye and Widow, the very people that Good Nebula tried her best to contact.
In terms of whether Good Nebula had the opportunity to use the time-space GPS after being beamed up to Thanos' ship, that's unclear at best, since the first time we see her on the ship is when she's getting roughed up by Bad Nebula.