Quality or target size
Use the quality presets for everyday sharing (80% is visually lossless for most photos), or type a maximum size, such as 70 KB for a form that rejects bigger files. GustKit then searches for the highest quality that fits and only reduces the dimensions if it has to.
PNG screenshots
PNG is lossless, so the biggest savings come from saving it as WebP or JPG. Choose the output format to do that. If an image can't get smaller in its own format, GustKit keeps the original instead of giving you a bigger file.