It depends
Buckwheat itself is gluten-free, but most commercial soba is cut with wheat flour for texture. Only noodles labeled 100% buckwheat, or juwari soba, are safe.
Japanese noodles made from buckwheat, which despite the name is not wheat and not a grass — it is a seed related to rhubarb and sorrel, and it contains no gluten. The difficulty is that buckwheat flour on its own barely holds together, so almost all commercial soba is cut with wheat flour to make a dough that can be rolled and sliced.
Because buckwheat is correctly on every gluten-free list, and soba is correctly described as a buckwheat noodle, and the two facts together produce a wrong conclusion. Somebody who has learned that buckwheat is safe orders soba and is eating wheat.
Buy juwari soba and cook it separately — it is more fragile than ordinary soba and the cooking water from a shared pot is its own problem. The buckwheat entry in this lexicon exists specifically so that matching on the letters w-h-e-a-t does not flag buckwheat; this entry exists so that being right about buckwheat does not make you wrong about soba.
Because the answer depends on the rest of the label, scanning the actual product is the only way to resolve it — CiCi checks for a gluten-free claim, the allergen statement, and which agency regulates that label, then tells you which way it lands.
Every one of these is the same ingredient. The number of names is a large part of why gluten slips past people who read every label.
Point CiCi at the barcode on the product you're holding and get this same reasoning in about two seconds, plus the carbs and the full nutrition panel. 360,000+ packaged foods are already read and waiting.
This answer comes from CiCi's ingredient lexicon — the same rules the scanner uses, not a separate article. Where reputable sources disagree, CiCi takes the conservative position and says so.