Start hugin
File -> Apply Template
Choose template project
select c2e.pto
Add Images
select images
as example:
cubeface_b.tif
cubeface_d.tif
cubeface_f.tif
cubeface_l.tif
cubeface_r.tif
cubeface_u.tif
Order is important! make sure that files are sorted by name!
go to tab Stitcher
set Canvas Size: width from the original Equi, usually width=cubeface*pi
Click Stitch!
hugin asks you to save project
and ask for the file name for the images
hugin generates 6 tif images with alpha-channel
then drag and drop these images "one by one" in gimp
then Image->Flatten Image
and File -> Export