Create a Sierpinksi Triangle using 0L-System with Python

I need to create a Sierpinksi Triangle using 0L-System with Python. See here: enter image description here I'm stuck here, I can't complete the triangle.

My code:

from turtle import *


def sierpinksi(size: int, n: int):
    if n == 0:        
        for i in range(0, 3):
            forward(size)
    else:

        sierpinksi(size//2, n-1) #F
        right(120)               #-

        sierpinksi(size//2, n-1) #G
        left(120)                #+
        sierpinksi(size//2, n-1) #F
        left(120)                #+
        sierpinksi(size//2, n-1) #G
        right(120)               #-
        sierpinksi(size//2, n-1) #F


sierpinksi(100, 2)
done()

My result: enter image description here

We can put a band-aid on your code by finishing the triangle after the fact and using floating division instead of integer division:

from turtle import *

ANGLE = 120

def sierpinksi(size, n):

    def sierpinksi_recursive(size, n):
        if n == 0:
            for _ in range(3):
                forward(size)
        else:
            sierpinksi_recursive(size/2, n-1)  # F
            right(ANGLE)                       # -
            sierpinksi_recursive(size/2, n-1)  # G
            left(ANGLE)                        # +
            sierpinksi_recursive(size/2, n-1)  # F
            left(ANGLE)                        # +
            sierpinksi_recursive(size/2, n-1)  # G
            right(ANGLE)                       # -
            sierpinksi_recursive(size/2, n-1)  # F

    sierpinksi_recursive(size, n)

    for _ in range(2):
        right(ANGLE)
        forward(size * 3)

speed('fastest')  # because I have no patience
sierpinksi(100, 3)
done()

But I believe the real problem is you've not embraced the spirit of the L-system, ignoring the starting production 'F-G-G' and not implementing the production rules. Your code does a lot of redundant drawing which makes it relatively slow. If instead we implement the L-system head on, we address all the issues:

from turtle import *

ANGLE = 120

RULES = {
    'F': 'F-G+F+G-F',
    'G': 'GG',
    '+': '+',
    '-': '-',
}

def sierpinksi(string, size, n):
    if n:
        string = ''.join(RULES[c] for c in string)
        sierpinksi(string, size/2, n-1)
    else:
        for c in string:
            if c in 'FG':
                forward(size)
            elif c == '+':
                left(ANGLE)
            elif c == '-':
                right(ANGLE)

speed('fastest')  # because I have no patience
sierpinksi('F-G-G', 300, 3)
done()