If you have 7 "Crystal Beast" cards with different names on the field or in your Graveyard: Special Summon 1 "Rainbow Dragon" or "Rainbow Dark Dragon" from your Deck or Graveyard, ignoring the Summoning conditions.
These are the only Crystal Beast monsters and they also total only 7 different names, meaning that if you want to use Rainbow Gravity you would need to add at least one copy of each of these Crystal Beasts to your deck.
You could use That Grass Looks Greener in a 30 card deck to try and mill out all the necessary Crystal Beast monsters to fulfill Raibow Gravity's activation condition. To take this further you can use the skill Meet My Family to create a 38 card deck, milling more cards with That Grass Looks Greener.
Special Summons from your Deck / Special Summons from your Graveyard / Ignoring the Summoning conditions
The problem isn't the skill.
RD must be summoned once at least before you use this card to summon it again from GY.
So, when you have RD in the hand, you shouldn't choose to discard it away if you don't have another copy in your deck.
If you use the family skill with Jesse your deck needs to be clean of crystal monsters. The rainbow dragon from the skill CAN ONLY BE SPECIAL SUMMONED with the 7 other crystal cards from the skill. I used RG with crystal deck with the skill and the RG card worked but i could not activate the cards own effect of special summoning it because the 7 different crystal cards were my own and the skills
...That is straight-up wrong. (And yes, I just wasted two minutes of my time testing this in the off chance it did happen to be bugged or something. I had a combination of my own and skill-added Crystal Beasts, worked fine.) All your Rainbow Dragons were probably in the Graveyard without having been properly Summoned first.
Semi-Nomi rule basically states that a monster that can only be special summoned must first be special summoned properly (by the listed effect) before it can be summoned from the graveyard/banished zone.
TL;DR, summon Rainbow Dragon properly first.
You cannot use this card to complete the mission of summoning rainbow dragon, i've tested it
Ignoring summoning condition just means you can revive a monster that normally cannot be revived.
However, ignoring summoning condition also means it is not a proper summon, therefore the monster must be properly summoned first (using the way written on the monster's text) before it can be revived.
Hey if they do give us this eventually its cool if they don't whatever