I think the reason Gnome Rider isn't used is that it takes up a spot in your deck, a spot that's always better occupied by something else. Many people doubt the use of Antlion for the same reason, though with its ability to burrow, that has some use. Gemfinder synergizes well with Stone Skin and escapes Earthquake, but Rider is just a wasted slot in an element with much more effective creatures.
A cost of 2
![Earth :earth](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/earth18x18.png)
would mandate comparable pre-doubling stats to Antlion, which starts 2|3 and becomes 4|4. But reaching 8|8 with only 3
![Earth :earth](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/earth18x18.png)
is unbalanced. "Minute" adjustments are pretty rocky when you're talking about low-cost stuff... we're talking about a 100% cost increase here, while bumping the cost of something from 8 to 9 means only a 12.5% increase. The cost-to-damage ratio changes dramatically.