I agree with startpacking, after 4pm there will be nothing to see at La Boqueria. If you want to visit the market, you should go there on Friday morning.

With your list of sites to visit, I would do the following:

Thursday: visit Sagrada Familia, then go to Parc Guëll. I don't know if they close before or right after sunset, but I would try to be there as close to closing time as possible. (Don't make it too close, it will get dark sooner than you'd expect because the sun goes down behind the hills behind the city.)

Friday morning: go for a walk from the harbour along the Rambla (and La Boqueria), a quick detour through the Barri Gotic, past Plaça Catalunya, up to Passeig de Gracia (try to go at least up to La Pedrera).

If you still would like to go to the park in the morning: I use the entrance on top of "Baixada de la Glòria". (There are escalators to bring you up there from Avenida de Vallcarca.) And yes, I have entered there before the official opening time (but don't remember what time that was), but I can't guarantee that the gate will always be open of course.