Gern(e) comes before the thing you like... what that thing is depends on you.
Ich liege im Sommer nach der Arbeit mit einer Flasche Bier im Park.
Here are your possibilities with the corresponding question you are answering.
Ich liege gern im Sommer nach der Arbeit... (what do you like doing)
Ich liege im Sommer gern nach der Arbeit... (what do you like doing in summer)
Ich liege im Sommer nach der Arbeit gern mit einer Flasche Bier im Park (what do you like doing in summer after work)
Ich liege im Sommer nach der Arbeit mit einer Flasche Bier gern im Park. (Where do you like lying in summer after work with a bottle of beer.)
Except for the last one, all sound smooth and the difference in perceived meaning is rather small. Which version people prefer is in my opinion mostly a question of rhythm and style... gern sounds very positive and it changes meaning after all... so bringing it right away results in a lengthy description after it that might eventually get boring... we already know that you like it so stop getting more and more precise. The very late gern however makes it sound as if there are other places where you don't like to lie. I think I would go with the second, but that has nothing to do with right or wrong.