new account for GMail only I added a second account in the GMail aplication, in order to be able to send/receive mail using that account.

I realized that the account has been added not only for GMail, but the Market (Play Store) also sees both accounts now. 

Is there a way to keep the second account only in GMail?
You can't do this because:


There's only one Google Services Framework running for all accounts.
Google Accounts credentials are stored centrally from where Google Apps authenticate.


But, there's a way to solve your issue: Turn on IMAP from Gmail settings. And, use IMAP in default Email app or K-9 Mail app. Done!
Play Store can't see it..