No, this is not recommended to be used on gutters that have cracks; this is a flashing kit.
When installing the Underlayment at a minimum of 45 degrees, it will immediately adhere to the roof deck. However, you can install it under 45 degrees, but it will not adhere until the temperature rises to 45 degrees or greater. I hope this is helpful to you. Thank you for choosing VELUX!
If you can get it in there so that it's properly covering what needs protecting, yes. But in order to do that, you'll need to remove the window.