Gambit
 

I want to select one line but all the lines of of the drawing are selected when I click on the drawing. I don't know why ?

did you import your geometry?
a picture may hel to understand your issue

I did not import any geometry. It happens when I want to mesh the edges one by one. It picks all the edges at the same time instead of one.

If you go to Geometry/Summarize edges, are you able to pick just one edge, or gambit selects again all the edges?
In other words, did you merge all the edges?