Quote:....I need to leave image2 with the same opacity as the example of image 1 .....
OK, got it (I think)
You can try matching using sample points see: https://docs.gimp.org/2.10/en/gimp-sampl...ialog.html
Any difficulty is assessing matching sample points between images. In this example I chose the dark shadow under the collar.
Then it is applying the curves tool, in alpha mode and getting an alpha value in (2) to match the alpha value in (1)
45 second example: https://i.imgur.com/JCgnduR.mp4