I feel it should only be allowed to grow a few times, like 2 or 3 times.
I personally feel there does not need to be a growth limit. While the Growth Ability is useful, you have to pay the card's ability cost over several turns before you actually get a profit:
Assuming you activate the ability as soon as possible and don't spend on anything else:
| Turn | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Quantum Spent | 0 | 0 | 2 | 2 | 2 | 2 | 2 | 2 |
| Quantum Gained | 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Net Benefit | 1 | 1 | 0 | 1 | 2 | 3 | 4 | 5 |
Since you spend 1 turn with a net benefit of zero from production, you don't actually start profiting until you use the ability 4 times. Upgrade gives you 1 extra quanta to negate that 1 turn of net zero, so it would take 3 uses to start profiting with the upgraded version.
Some other factors that you might want to consider:
This is a mark - assuming that these cards are a result of The Trial most players will only have 1 copy. It's not spammable like a pillar.
Gravity has no quanta acceleration method (compare Fire and Entropy, who have Immolation and SNova which have made decks involving either very fast and quick damaging.) and is generally considered weak in mono. While this mark is quanta acceleration, it takes at least 2-3 turns to get it effectively running, whereas 1 succesful Immolation or SNova can result in at least 1 nonpillar card being played immediately after it.
