Is there a way to be only online for some friends in GChat?

Its kind of yes and no answer…
Yes, you can set your status to appear invisible when using the Google Talk gadget or Gmail’s chat feature. While signed in as ‘Invisible,’ you’ll appear offline to your contacts.

To go invisible, select Invisible from the status drop-down menu.

If you don’t want a particular user to be able to see your online status, you can block that user. Once blocked, though, you won’t be able to see their status, and you won’t be able to communicate with them on the Google Talk network.