Gecko is the browser engine, networking, parsers, content models, chrome and the other technologies that Mozilla and other applications are built from; that is, the parts that are not application specific.
See https://developer.mozilla.org/en/Mozilla_Embedding_FAQ/Introduction_to_Gecko_and_Embedding
Many of the changes made for LiveCycle ES2 (9.0) are related to AIX and Solaris. For AIX, because Gecko cannot be compiled using GCC (http://www.mozilla.org/unix/aix.html), Adobe used already-built Gecko libraries, in which some functions are not present. After confirmation that a function was not needed, Adobe commented out that call.
Details are outlined in the Gecko Changes document.
