This will vary from person to person. We all absorb carbohydrates at different rates but regardless of this the process is the same.
It starts with amylase. It's an enzyme that's found in your mouth and whose job it is to break down the more complex carbs found in the gel. After this another enzyme, pancreatic amylase, takes up the slack as the gel moves into your small intestine. After that it's down to transporter proteins found in the gut. They absorb the glucose and other sugars, helping to pass them into the blood stream.
Fortunately gels are designed to act quickly, delivering you the energy you crave in quick time - this could translate to a few minutes for some or perhaps 15 minutes for others.
There are usually simple sugars in there, alongside the more complex carbs like maltodextrin, which don't require enzymes to break them down before they're absorbed by the gut, giving you a mix of a quick sugar rush and a longer energy flow from the more complex carbs.