After having converted the grease pencil strokes to a mesh I can't add any texture in texture paint mode I've watched this tutorial about grease pencil in blender 2.8 by Paul Cageggi and I've got the idea to apply over the car a texture of a real car found on internet using the projection painting tecnique. 

In Object mode, with the stroke object selected I hit the W key and then I selected the option "convert to mesh",as suggested in this post. 

As soon as I have converted the strokes to a mesh,I have switched from layout to texture paint and then I tried to add a texture,as suggested by this tutorial,but surprise,I can't do that. I have no "texture paint" option and I can't add any texture,as you can see below. Can you explain to me why ?
I don't think you can directly paint onto the GPencil object that you are converting to mesh. I include a picture here of converting GP to bezier and then to mesh once closed.

Texture paint mode is only available for Mesh objects, so that is why the option doesn't appear.