Here is a list of how I know/think Gajeel likes Levi.

1) He saved her from Laxus when he attacked her.
2) He "volunteered" to be her partner in the S-Class games.
3) He helped her up a cliff during the S-Class games.
4) He saved her from the goons from Grimoire Heart.
5) He was with her when they were found after 7 years in Tenroujima Island.
6) He told her that he won't dance with her when Lucy was teaching Natsu how to dance, but while Jet and Droy were arguing about who was going to dance with Levi, Gajeel said, "I guess I have no other choice but to dance with you."
7) While everyone was getting ready for training before the Grand Magic Games, Gajeel told Levi not to come with him and Lily for training, he secretly said, "I don't want you to get hurt.
8) During the "Modeling" match between Mirajane and Jenny in the GMG(Grand Magic Games) in the wedding portion, he said he would take a nap, disappointing Levi, he was thinking, "I don't want to embarrass Levi and myself, but she does look hot in that dress!!"

If you have or know anything that I might have missed, let me know.