UX

It is not obvious what the user should do when the GUI opens up. You should display some simple instructions in the GUI, such as:

Use arrow keys on keyboard to move right, left, etc.

When I exit out of the GUI window, I see several errors in the shell in which I ran the game. Perhaps this is specific to the version of Python I am using.

Efficiency

In the snake_move function, the 4 if statements should be combined into a single if/elif because the 4 conditions are mutually exclusive:

def snake_move():
    if snake.direction == "up":
        y = snake.ycor()
        snake.sety(y + 20)
    elif snake.direction == "down":
        y = snake.ycor()
        snake.sety(y - 20)
    elif snake.direction == "left":
        x = snake.xcor()
        snake.setx(x - 20)
    elif snake.direction == "right":
        x = snake.xcor()
        snake.setx(x + 20)

You should also be able to eliminate the intermediate variables. For example:

    if snake.direction == "up":
        snake.sety(snake.ycor() + 20)

Magic number

The 20 in the snake_move function should be a declared constant. You should also add a comment describing why you chose this value.

DRY

The 4 functions:

def snake_go_up():
def snake_go_down():
etc.

could be combined into a single function with an argument passed in corresponding to the direction.

This group of lines is repeated twice:

time.sleep(1)
screen.clear()
screen.bgcolor('turquoise')
scoring.goto(0,0)
scoring.write("   GAME OVER \n Your Score is {}".format(score),align="center",font=("Courier",30,"bold"))

They can be placed into a function.

Layout

Move the functions to the top after the import lines. Having them in the middle of the code interrupts the natural flow of the code (from a human readability standpoint).

There is inconsistent whitespace around operators, such as

            a= snake.xcor()
            b = snake.ycor()

The black program can be used to automatically format the code to add consistency. This will also help with the inconsistent indentation and excessive blank lines.