From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Gajira Norimaki (則巻 ガジラ, Norimaki Gajira; FUNimation "Gajira "Gatchan" Norimaki", Viz "Gadzilla "Gatchan" Norimaki") is actually the name of two identical green-haired, Angels. Originally, one hatches from an egg brought home by Senbei Norimaki after a time traveling trip. Later on, Gajira inexplicably replicates a second Gajira copy. The Gajira eat almost anything, except for rubber, and particularly like to eat metal. The Gajira could also shoot rays from their antennae. They speak in their own language that consists primarily of sounds like "pupipopipi" that Arale Norimaki somehow seems to understand.
It was revealed in Volume 17 that Gajira was actually an Angel sent by the god of the galaxy, or Kami if this story takes place in the Dragon Ball universe, to prevent further development of the Human civilization. After seeing that other civilizations eventually destroyed themselves and the planets they lived on, Kami placed an Angel's egg upon Earth. Gajira's ability to replicate as well as its fondness for eating metal should have ensured that humanity would remain primitive and innocent. Seeing that Gajira failed in the mission (due to Senbei's time traveling intervention), Kami was about to destroy humanity. After seeing the life of Penguin Village, however, he changed his mind and left Earth as it was.
 Dr. Slump Arale-chan
Gajira is the combination of the names of Gamera and Gojira, the latter known as Godzilla in the West. That is why they are known as Gadzilla in the English manga.
 Dr. Slump
Gajira came out of a strange looking machine instead of an egg. Also, the Gajira had blonde hair instead of green.
 Video games
Gajira No. 1 and Gajira No. 2 are featured in one of Arale's attacks shooting the enemy and saying, "The Gatchan are really strong too" in Dragon Ball Z: Budokai Tenkaichi 3.