How do I block someone from knowing if I'm online in Gmail?

In the chat window, hover over that colleague. When the window pops up, click the down arrow in the lower right corner and select “block”.

NOTE: If the person is not in the 5 or 6 people shown in your chat window, simply start typing his/her email address in the “search people” box. Once this person appears, hover over it and you should have a “block” option.

Ah – that’s terrific! Thanks!

I bumped into this person and she said, “I saw you were doing Gmail this morning.” Creeped me out.

If you ever want to be invisible to everyone, click next to your own name and choose “invisible” for your status.

@augustlan Yes, I know that one. But I want to be visible to most people this time, just not her.