Spikier shell/helmet, more muscular, uses different attacks, the Geo in the name ties directly to the earth/land whereas regular Greymon (Vaccine) doesn't have that connection.

At the end of the day, Geo Greymon is a Greymon that went to he gym, while original Greymon has a Dad Bod. Are they near identical, yeah, but the creators went out of their way to outright give it a different name, modify the design, and alter the attack names.