skinning helmets - transparent visors GIMP 2.10

Hello guys!
I'm trying to skin a retro car in race 07 (volvo amazon) and i have some problems with helmet type2 template.
Using gimp 2.10, i can obtain only 100% opaque visors. Is there something that i dont understand with layers, alpha and channels?
Someone can help me? i have successfully skinned interior and exterior windows of the car, so the basic idea of how to obtain transparency is clear but... with helmets not so much..

greetings

Ps i have found an old thred here on racedepartment about this problem, but it was created in 2014 (a lot of necroposting..)
 
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