Do you have a documentation page where it lists all the Groovy methods I can use as a Gloop step? I am trying to look for a method for encoding and decoding to/from base64.
Going back to your question, Martini also ships with the default Java/Groovy library (and more). For example, when trying to find something, you can press the dot key anywhere in the Gloop service editor, start typing, and it will populate stuff for you.
If you are coding a Groovy script, you can use control+space to open Groovy proposals.