I am trying to paint this self designed thermos bottle glossy red but I am not been able to understand the light and how to give the glossy effect and how to give light effect on the division line in between bottle.
I will cheat and give you some links for you to study.
What you will notice and learn is that a glossy material has its looks for what is what they are reflecting, not what the material is made of.
And here is an example applied. In that post you will see how "glossy" means reflection.
And let's get into your image.
I just added some random shapes with a gradient fill inside and transparency.
They try to show what was the light's shape and where it was when you took the image.
Here you can see the shapes.
Here I am playing with some gradients.
On the first image, I have a gradient similar to your image.
But see what happen if I start playing with the relationship between the light and dark zones.
Here I added more white. Not it is not only a gradient, it has a big source of light illuminating it.
If I bring closer a dark gray, I am saying " the white is a light source reflected with a dark surrounding. My surface is glossy.
The closer this contrast is, the glossier it becomes. In 3D material design, this is called the roughness of the glossy material.
This is a "spike" in the shading, in the gradient. It can be used for example to make human skin look metallic.
Take a look here.