Can't join two point (one is not an ending point)

You are welcome, pleymo.

I had to leave for a while when answering your question; and saw that it was unnecessary.

But you may like to use the Opacity Mask way in other cases, so it may be worth reading anyroad.

pleymo,

There was an incomplete bracket to make it even more unintelligible, sorry.

Here is a hopefully clearer explanation:

1) Select all the paths and Ctrl/Cmd+G to Group them and set their Stroke to black;

2) Create a background rectangle (or whatever) with the desired gradient and Ctrl/Cmd+X+B to bring it to the back, so it is behing the paths from 1);

3) Select everything and in the Transparency palette (flyout) click Make (Opacity) Mask with both Clip and Invert Mask ticked (in the main Transparency palette), see help page and explanation below.

With these settings, the parts of the rectangle behind the paths will show and the rest will be hidden, so it will appear as if the paths have the gradient. This is the way the Opacity Mask works. You can read more about it here:

https://helpx.adobe.com/search-results.html?q=make+opacity+mask&scope=%5B%22helpx%22%5D&subscope=%5B%5D&limit=10&start_index=0&sort_orderby=relevancy&sort_order=desc&post_facet_filters=%7B%22applicable_products%22%3A%5B%5D%7D

You can also use the helpx pages as a source for many other things. Here is an empty search page you can Bookmark, and then just fill in the search term:

https://helpx.adobe.com/search-results.html?q=&scope=%5B%22helpx%22%5D&subscope=%5B%5D&limit=10&start_index=0&sort_orderby=relevancy&sort_order=desc&post_facet_filters=%7B%22applicable_products%22%3A%5B%5D%7D

Thanks to everyone !

I have made this shape and I want to add a color gradient  effect but if there is lines which are not joined, the color gradient effect will not be global to the whole shape.

How will you do that ?

What is the purpose of your request?

Perhaps there are alternative approaches to do what you want to do.

Any anchor point can only be connected to 2 other points. It is already so joined, so you can't do so here, there are no 3 point joints in Illustrator.

No. You can only join endpoints.