I think it’s because you are working very light and using lots of water with you gouache. If you use it like watercolor, it tends to be extremely streaky. You can start with a thin wash but you should end up with thicker paint. You want to build more layers and use less water as you go. It also has to do with the paper. I see that you are using a sketchbook. The paper tends to be more absorbent. It is much harder to spread color across the page on an absorbent paper because it just sucks all the water and leaves streaky paint. I would recommend watercolor paper if you are going to use gouache that is thinner. Something less absorbent allows the paint to glide across the paper.