Free shuttle bus from Metro
There is a free shuttle bus (Bus Güell) that connects the Park Güell with the unterground station Alfons X (L4). The bus ride takes 15 minutes from the underground station to the entrance of the Park Güell. There ride back from the Park Güell to the undeground takes also around 15 minutes.
You will find a blue bus stop next to the subway station. You will have to wait some minutes and take the first bus that arrive to the bus stop. Make sure to arrive early to be able to enter in your entrance ticket slot.
In the bus stop you can not buy an entrance ticket, so to be able to take the free shuttle bus you will have to buy it online. You can do it right now here: