Q:

What does 言っちゃえば mean? And what does 地面を蹴っていた mean?

I don't know what is the meaning of 言っちゃえば.
The sentence is the following:

言っちゃえばこれがダンジョンでの直接の稼ぎになる
"That is, this will become a direct gain coming from the dungeon."

Is it correct the meaning "that is"?
The other problem is "地面を蹴っていた"
The sentence is the following:

そいつの目とこちらの目があった時,　 既に僕は地面を蹴っていた
“When we found ourselves face to face, I already had my feet on the ground.”

Is the correct interpretation the his feet were already on the ground in that moment?

A:

言っちゃえば is a colloquial form of 言ってしまえば (see this if you are unsure). And ～ちゃえば/～てしまえば in this context is "If I dare ～." It implies the following fact (これが直接の稼ぎになる) is something he doesn't want to say too openly.
Using "That is, ..." is not bad, but I think "Frankly, ..." or "As a matter of fact, ..." would be a bit better.
地面を蹴る is an idiomatic phrase meaning "to jump" or "to start to dash".

