Gyros Thessalonikis is the Thessaloniki reading of the turning-spit sandwich, and the first thing to understand is a vocabulary quirk: in Thessaloniki this is very often ordered as souvlaki even though it is plainly rotisserie meat off a cone, not skewered. That naming split is its own subject and deserves its own article rather than being crowded in here. What matters for the sandwich itself is that the northern style runs to larger portions and a slightly different seasoning than the southern build, and it is widely held in Greece to be among the country's strongest gyros, which is a claim about execution and proportion rather than a different machine.
The build is the standard Greek sequence with the Thessaloniki accents in scale. Marinated meat, most often pork and commonly chicken, is packed onto the vertical spit in overlapping layers and roasted as it turns; the cook shaves the crisped outer edge in thin slices, because thin is what crisps and thick stays pale and chewy. The pita is brushed with oil or fat and griddled, sometimes against the spit drippings, until it is soft and pliable with a few blistered spots. Then it is built hot and fast: meat, tomato, raw onion, tzatziki, and fries rolled tight in paper. Because the northern portion is generous, proportion is the thing that separates a good one from a sloppy one; a heavy fill demands a pita that is genuinely soft and well wrapped or the whole thing splits and spills. The universal failures still apply: lukewarm meat off the spit, a stiff cold pita that cracks, watery tzatziki that soaks the bread to mush, underrendered pale slices. Done right it is a big, tightly wrapped sandwich that still reads layer by layer despite its size.
It shifts mainly against its siblings and its own maximal form. The Athenian build on a smaller pita, the lamb cone, the mixed-meat version, and the plated merida served without bread are each their own preparation and treated separately. The fully loaded apo ola reading of this same Thessaloniki style is close enough to warrant its own entry as well. The constant here is the northern logic: a generous cone, a soft well-handled pita, the meat shaved thin and built hot, with tzatziki and tomato cutting the fat across a larger sandwich.