Question

Was lernst du gern in der schule? (warum) Was lernst du nicht gern? (warum)

Answer 1

Was lernst du gern in der schule?
What do you like to learn in school? (why)

Was lernst du nicht gern?
What do you not like to learn? (why)

Gern is used along with another verb to make "to __" into "to like to __."

An answer would look like:

Ich lerne gern Mathe in der schule (I'm not very good at speaking German, take
this with a grain of salt.)

Answer 2

One example for my case: Mein Lieblingsfach ist Physik aber ich mag Deutschunterricht auch ganz gern (my favorite subject is Physics but I also like German class).
In simpler that would be: Ich lerne gern Physik und Deutsch (I like to learn Physics and German).
And with reason: Ich lerne gerne Physik weil mich Naturwissenschaften interessieren und ich einen tollen Lehrer habe. (=I like learning Physics because I'm interested in science and I have a good teacher).
Hope that helps ;)