Piloted Gundam-like mecha? All Walkers I know about are two or multi-legged non humanoid models.
First thing that comes to mind when concerning Gundams and Star Wars is that Transformers+Star Wars toy line from many years back which weren't really part of a story or anything, just some neat toys.
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The Millennium Falcon Transformer (from 2006) was sort of gundam-ish and could split up to transform into Han and Chewie. But its not part of the SW universe obviously.
As for in-universe, closest thing to looking like Gundams or mechas in general would be some of the mechanized Zakuul machine gods but those weren't created until 2015 for TORtanic, just before TFA came out. But they're not mechs since they can't be piloted.
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It comes from planet Zakuul and fitting to the name of its planet, it looks like a Zaku mobile suit. Kinda... Sorta reminds me of a Z'Gok.
There were also battle droids that could be piloted, but none of them resembled gundams or traditional mechas as far as I know. There were the SD-series droids which were the largest of their kind but they couldn't be piloted I think.
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The SD-9 in particular looked like an RGM mobile suit but it wasn't too big.
But a very interesting mecha, at least for me, was the Basilisk War Droid which was used by Mandalorians, although it was more of a small zoid than a gundam.
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Or perhaps it was this thing you saw?
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The Great Heep was a villain in the 80s Droids cartoon. It was a giant droid that could potentially consume and grow endlessly.