Is something like the Java API or comprehensive manual of all already implemented methods for Forth?
There is not a single source defining/describing Forth words (methods) because different implementations of Forth have different sets of implemented words. Not only do different Forth implementations include different words, but the behavior of the words may be different across the different implementations. Several Forth standards have been specified including FIG Forth, Forth-79, Forth-83, and ANSI Forth (1994). You can find a copy of the ANSI Forth specification at http://lars.nocrew.org/dpans/dpans.htm.
Gforth is a specific Forth implementation and is compatible with the ANS Forth standard. You can find more information including a manual with a glossary of defined words at http://www.gnu.org/software/gforth/.