If you use the command /clearworld, with the preset NOT_ORE, like so, /tfc clearworld 100 NOT_ORE, then all non-ore blocks are deleted, but gravel is also not deleted for some reason, even though the only blocks not deleted should be ores..

NOT_ORE dynamically checks what blocks spawn as an vein feature; gravel spawns in a vein feature, so no, you are wrong.