You can do better than that for a source. http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%... :

I. earth opp. to heaven, or land opp. to sea; κατὰ γῆν on land, by land

2. earth, as an element, opp. to air, water, fire

II. a land, country; γῆν πρὸ γῆς from land to land

III. the earth or ground as tilled

IV. a lump of earth, in the phrase γῆν καὶ ὕδωρ αἰτεῖν, γῆν καὶ ὕδωρ διδόναι, in token of submission

http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%... is a much more detailed, less quotable dictionary entry.

As you can see, the greek word refers equally well to the planet (sense I, "the earth, including land and sea, in contrast to heaven") and the dirt (senses 2, III, and IV), just like the english word.

So it only breaks down on gas giants ;-)