Alright. I should preface this with two things:

First, I’ve tried a few times in the past to research this. There’s lots to read about the language, but less about the history. I couldn’t find anything at my then-campus library the first time, nor at either of my huge regional libraries the second time. One of the few scholarly sources I found is a paper (or an interactive Ivy League Thing), linked below, which I originally found as a citation at Wikipedia under an otherwise poorly-cited article. Maybe Wikipedians are having as much trouble as I am =P

I’m going to lean fairly heavily into this source, as it speaks most directly to what Gullah slaves were doing on those plantations.

Second, I’m gonna have to talk dispassionately about African slavery in the United States.

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African rice is not like Asian rice. They’re related, but they aren’t the same. African rice was historically cultivated along the west coast of Africa, especially Sierra Leone, and the people growing it had a certain amount of expertise. Think about rice cultivation - has almost nothing to do with wheat or cotton farming!

So when it was discovered that African rice would grow off the SE coast of the United States, slavers began importing slaves with experience in rice cultivation or the related infrastructure.

Although many American plantations existed with majority-black populations (mostly or entirely enslaved) it’s difficult to express just how stark the contrast was on these rice plantations. We’re talking about many hundreds of Africans and their children on island plantations - from all over Africa, but especially from tribes native to Sierra Leone - and that simple fact began the process of creolization. Contrast that with the experience of your average black slave on the mainland, where the only thing you were sure to have in common with the person next to you was being enslaved in and of itself.

Summarized by Yale:

The connection between the Gullah and the people of Sierra Leone is a very special one. Sierra Leone has always had a small population, and Sierra Leonean slaves were always greatly outnumbered on the plantations by slaves from more populous parts of Africa—except in South Carolina and Georgia. The rice plantation zone of coastal South Carolina and Georgia was the only place in the Americas where Sierra Leonean slaves came together in large enough numbers and over a long enough period of time to leave a significant linguistic and cultural impact.

Now fast forward to the Civil War. You have something more or less resembling Gullah people, and a handful of white slavemasters and overseers, and the islands export rice. Then the Navy shows up to blockade the South. The white people flee to the mainland, leaving islands full of nothing but Gullah people, rice farmers mainly, and nobody else. These would later be among the first people formally liberated by Union forces, and they remained the primary occupants of the Sea Islands.

Those ongoing circumstances, an unusually large number of specialized slaves from a specific geographical region, the benefits of staying in one place, the early departure of slavers who left their slaves behind, the intervention of the Union who mostly left the ones alone who didn’t join the famous South Carolina colored regiments, and then another century of general isolation, created a Gullah identity. And here they are today, a slave-descended people off the southeastern coast who’ve retained a creole language, a unique culture, and even some terminology and songs and etc. that go all the way back to Sierra Leone.

Main source: Yale, The Gullah: Rice, Slavery, and the Sierra Leone-American Connection, archived here