Not an expert but this is my understanding.
All children use chunks, not just GLPs - it's a very normal part of language development.
The difference between typical language acquisition and GLP as I understand it is that GLPs go through stages in a different order.
Typical language acquisition will be:
Single words (and some chunks but they think of them as words e.g. "where is it?" or "All done" or "Bye bye" or "Thank you" or even phrases like "Paw Patrol is on a roll")
Combining 2-3 words together (Daddy + home) = "Daddy home?"
Combining words in a more flexible way e.g. "Daddy home now?" "Daddy comin home later."
The words will often be simple and familiar - they will mimic words they hear often but they are less likely to pick up on individual words that aren't commonly used around them. Because when they hear other people speak, they recognise that there are individual words within that stream of talking.
Whereas GLPs don't have that recognition to begin with - they hear a sentence and they process it as a whole. So they might not go through those stages - instead they will have some longer phrase which has meaning to them, and it might contain quite obscure or surprising vocabulary that you wouldn't expect to be familiar.
They then have to move through phases of breaking the gestalt down, mixing + matching it, then finally breaking down more and getting to single words before being able to put those back together.
Sometimes they manage this fine and you might not even realise they are a GLP - I think DS2 (5) is possibly an example of this, no professional thinks it is remotely interesting BTW but I'm fascinated!
But some DC (commonly on the autism spectrum) can get stuck in stage 1 and struggle to move forward from there. It seems like the GLP tips you can find online about modelling useful phrases and trying to track any meaning and context you can find for their gestalts are harmless if they aren't GLP, and could help hugely if they are, so may be worth trying?
This is useful.
https://www.meaningfulspeech.com/blog/stage-1-stage-3