Hello,
I have designed a component (which is a antenna bluetooth printed in the board) to be placed in the board.
I want to do it in the same way of the next picture:
That is, cooper area, but the gerber can't say this area will have tin (lead free). I mean, I wan't the stencil to have this are routed.
How can I keep a copper area visible, and avoid this mark in the stencil?
Thanks in advance
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
Not sure we understand your question.
Are you asking how to make the copper track that forms the antenna so that:
1. there is no solder mask put over it and therefore;
2. the copper is exposed and so;
3. is plated?
If yes then please see the FAQ:
https://docs.easyeda.com/en/FAQ/Editor/index.html#How-to-add-solder-mask