or for moving a image that is in simple cylindrical to North Pole and South pole
and back again ! gprojector can NOT do that

Gimp has a built in tool called polar projection
remap the north and south half into a polar projection
work on it
then remap it back into simple cylindrical

for example :
this image from above , not quite in Simplecylindrical looks to be a bit more Mercator
that needs to be converted
( geo-referenced to 74 n to 74 s and remapped )


then grab just the south and stretch it
then remap it
/filters / distorts / polar coordinates ( to polar )


and work on it ( the plate at the pole is a bit "scrunched" )


then remap back to simple cylindrical
/filters / distorts / polar coordinates ( from polar )
and past it back into the original image