Tilesetize - each top level layer in the project will be exported as a tile. Resulting tilesheet will be as close to square as possible. JSON annotations have specific format.

Spritesheetize - each top level item in the project has to be a layer group (= animation clip). Each layer in animation clip is an animation frame. Each clip will always be on a single row of resulting spritesheet, there may be multiple clips per row (there is simple packing algorithm going on). JSON annotations have different format (obviously, you need to annotate multiple clips). Name of the animation clip in Gimp will be used in the annotation.

You could pretend that set of tiles are just another animation clip and export them as a part of the spritesheet. In that case, you would get quite long image (depending on number of tiles) because the packing algorithm won't wrap it.