As almost all objects in GAP, transformations, too, are representend by records. Such a transformation record has the following components:
isGeneralMapping:true, since in particular, any transformation is a 
  general mapping. 
domain:Mappings.
isMapping:true since a transformation is in particular a single
  valued mapping.
isTransformation:true for a transformation.
isSetTransformation:true for set transformations exclusively.
isGroupTransformation, isGroupElement:true for group transformations 
  exclusively.
elements:
source, range:
tfl:
operations:=, etc.
  can be found here.
image, rank, ker:GAP 3.4.4