The sudden 180s and different expectations from each character is the beauty behind their development. How they grow and change into someone they want to be is realistic, and it's a callout to the reader by asking them, "Would you still love and accept them for who they are?"
Lars is uninhibited and wild by nature, but he holds himself back to appear socially acceptable in a world that strictly binds behaviours like his. As a result, he does a lot of things that may be uncomfortable to him but he does it out of obligation e.g. Excessively clean, constantly doing kind things for others, suppressing his feelings, and being a Yes Man. The moment he realizes he has a space to let loose and a person who he can trust, he molts his shell once more.
Gil is the slight opposite of this. He's outwardly rambunctious and defiant of social standards, but he has grown to be uptight and reserved due to his upbringing. He's like a mirror, acting the way other people would want him to act. Hence why he often talks about his habit of masking himself and only allowing people to see the most fun and perfect side of him e.g. Acting like Edgar towards Walter, acting as the perfect superstar coach towards his students, acting as the rowdy teenager towards Parker and Lars, and acting as the uncaring sibling/son towards his family. His scenes are amongst the least explicit because he's mirroring Walter's desires: To be the caring, reliable figure that will keep him safe for life.
Both of them have a lot of overlapping themes, but they're more or less aiming for the same thing. The character you knew them as on Day 1 is drastically different by Day 12. If they don't live up to your standards, then that's how most relationships work out.