Plain GD (unless very ancient PHP) already works well with WebP and thumbnailing though. It's just config.php suggests IM and GM are still better at this job for some reason.
GD thumbnail functions are more hardcoded in image.php thus less configurable. I could modify it myself but I think it's better to suggest upstream changes rather than fiddling alone.
For example, what do you think about adding imageinterlace() for making progressive (smaller) JPG thumbnails, and imagejpeg() defaulting to about 75 quality?