Gravitino Balls are a rare Trade Commodity in No Man's Sky obtained via Farming Gravitino Orb plants at your Home Base or from the wilds in a few different forms.
Some will be a creature that extends a tentacle upward when you go near and others will simply be a lotus like plant with a much larger Orb sitting on top.
It's primary use is in the construction Theta level Cooling and Shielding enhancements.
Where To Obtain
Here is a planet where Gravitino Balls can be found in the creature form, extending a tentacle as you go near with a glowing Gravitino Ball at the tip, that only requires the first 2 Portal Glyphs to dial:
It is a very pleasant planet that really stretches it's Paradise label with being somewhat picturesque yet not all that well covered in Flora and Fauna.
This planet also plays host to Star Brambles.