You can find most popular coin collecting phrases in this coin glossary.
Gem - a beauty of a coin. One with unusually high quality.
Gem BU - A beauty of a coin ! Means GEM quality Brilliant Uncirculated coin. Indicates that this uncirculated coin shows mint brilliance and is extremely attractive for the type of coin. Some might say it sparkles like a "GEM". See Coin Grading
gouges - heavy marks on a coin where the metal was gouged out from coming in contact with something. Typically worse than "contact marks" or "bag marks".
grade - a rating or clarification that indicates how much wear a circulated coin has. Grades can also indicate the degree of perfection for uncirculated coins. Two popular grading guides are Photograde and the ANA Grading Guide. Both use a scale system from 1 to 70, measuring coins from About Good -3 to Mint State Uncirculated- 70. See Coin Grading
grading standards - a set of criteria indicating how much wear a coin shows.
gram - weight. Metric system of measuring weights. Approximately 31.1 grams are in one troy ounce.