I really do not advise using GAP unless you have absolutely no other option or are very curious.
It was originally for an older version of Gimp and now many of the functions do not work 100% (if at all) add to that a very steep learning curve, picky about layer names..etc..etc.
There are newer equivalents for parts of GAP. An example is the GAP morph plugin. That can be used on its own, but these days the G'mic interactive morph is an easier alternative.
Quick example:
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