The following sites have Yiddish dictionaries:

Harkavy Yiddish Dictionary  Yiddish Dictionary online

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Gabbai:  (Shamash, sexton, attendant or beadle) - An Aramaic word  that connotes 1. the treasurer/collector of  communal taxes or charities for the needy, a  of sorts. 2. One responsible for the smooth day-to-day running of synagogue logistics. In practical terms, the gabbai decides who leads the service, who should be called to the Torah etc.

Gadlus hamochin: Exuberant rationale.

Galus/Galut: exile,Diaspora

Gaon: The title of the sages who lived during the post-Talmudic period (roughly 7th to 11th centuries). This period is thus known as the "Gaonic" period. The title of one who is an especially learned scholar

Gadol hador: Greatest sage of any generation.

Gashmius: materialism, physical needs

Gedrungen: Proof

Gedulah ("greatness"): A synonym for chesed.

Gehennom/Gehenna: "hell"

Gemara; (Aramaic): Interpretations and explanations of the Mishnah; see Talmud.

Gematria: a system for calculating the numerical equivalence of letters, words, and phrases. This system is used for the purpose of gaining insight into interrelating concepts.

Ger Toshiv: A 'partial' convert who agrees not to worship idols

Gerah: A type of coin, equivalent to approximately 0.64 gram of silver.

Geulah: Redemption, usually referring to the Redemption through Moshiach

Gevaldic: Amazing (yiddish)

Gevalt: Amazing, sometimes an expression of shock

Gevurah: "Might". The fifth of the ten sefirot, and second of the emotive attributes in Creation.

Gevurah/Gevurot: lit. "power' or strict justice. The second of the seven Divine middos, or attributes, and of their corresponding mortal middos, or spiritual emotions.

Gezaira: decree

Gezeilah: robbery, theft

Gezeirat HaKatuv: (גזירת הכתוב) - a blical decree

Gezerah Shavah ("equal category"):  One of the thirteen hermeneutical methods of the Torah, in which two concepts or cases are compared on the basis of terminology common to both. An inference from the juxtaposition of Biblical verses

Gilgul/gilgulim: incarnation in a particular lifetime

Gimmel Tammuz: The 3rd day of the month of Tammuz, the day of the Histalkus of the Rebbe

Gimmel: One of the 3rd letter of the Hebrew alphabet.

Girsa: Variant reading or version. Brief Talmud study 

Gitten: Tractate of the Talmud

Glatik: (yid) Smooth, straightforward, ideal. e.g. The explanation does not entirely 'fit' 

Golah: Exile; Diaspora.

Gonif: thief

Gulgalta ("the skull" Aramaic): The keter of Arich Anpin. In psychological terms, the interface between pleasure and will, which serves as the origin of the super-conscious will.