They could be students in the same school. Gil might be good at math and helps Gail with her studies, while Gail is excellent in art and inspires Gil to be more creative in his projects.
I don't know of a 'Free Gil Brewer'. It might be a unique name that is associated with a particular story, place or activity. Without more context, it's difficult to say who this person is.
One idea could be that Harry and Gil go on a treasure hunt that was hinted at in Descendants 2 but never fully explored. They could encounter new magical creatures and face challenges together, strengthening their bond as friends.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
I'm sorry, I'm not familiar with a specific 'Free Gil Brewer' story. There could be many people named Gil Brewer. Maybe you can provide more context to help me answer better.
Sure. You can use GIL as currency in a story. Think about how it will function in the economy of your fictional world. For example, will it be a precious metal - based currency like gold, or something more abstract? How will it be exchanged? Will there be different denominations? You can really have fun with creating the rules around this fictional currency in your story.