Are all of these free to use?
According to the (old) tileset upload/download agreements, if you upload/post a tileset (or gradient) in the tileset section of the forum (as most of these were), then you are giving anyone the right to use your tileset (or gradient) in their level as long as they credit you.
Unless the author explicitly states otherwise (for example: AlmightyZenTaco's Inner City Knytt), tilesets only available in KS levels are off-limits without asking the author.