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.
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.