The Gigaverse is a huge "complex" of various Megaverses. The megaverses within the Gigaverse have less connections and are further away than multiverses in the megaverse and universes in the multiverse, which is why -verses leveled inbetween the Gigaverse and Omniverse start to lose any signifcant meaning. Due to the lack of any significant meaning, the Gigaverse is the start of the Archverse chain.
The high Kardashev Scale jump is because of the low density of Megaverses. For reference, a Megaverse scores a 6 on the Kardeshev scale.
Gigaverses aren't actually part of the normal scale of multiverses, but they still play a big purpose in existence.