How to adjust filling pattern

[snowforest]

If I want to apply gradient filling, it automatically starts from black and goes to white. But I do not want it to start from that dark. How can I adjust the start and finish colors of gradient fill? I see the box but it looks very complex and nothing happens no matter what I do and first of all the escape button doesnt work in gimp.

Once I am done with filling how do I get rid of dashed lines around the selected area? Escape button doesnt work