What the gamma symbol means
Gamma (γ, capital Γ) is the third letter of the Greek alphabet. The capital Γ is a simple vertical stroke with a bar on top, and the Cyrillic letter Г was adapted from it.
In physics, γ is the symbol for gamma rays, the most energetic form of electromagnetic radiation, and for photons in general. In relativity, γ is the Lorentz factor. In math, Γ(n) is the gamma function, which extends factorials: Γ(n) = (n − 1)! for positive whole numbers. Small γ ≈ 0.5772 is the Euler–Mascheroni constant.
Gamma turns up in many other fields too. Screens and photos use a gamma value to describe how brightness is encoded, and many displays target a gamma of about 2.2. Options traders use gamma for how fast an option's delta changes. In biology you will see gamma globulins (antibodies) and GABA, short for gamma-aminobutyric acid.
Gamma vs. look-alike letters
The Cyrillic Г (ghe), used in Russian and other languages, looks just like Γ, but it is a separate character. If you mix them, search and spell check will fail, so copy Γ for Greek and math. The Latin ɣ is the phonetic symbol for the soft voiced "g" heard in Spanish "lago", and ɤ is a different phonetic vowel.
Math also uses double-struck forms, ℽ and ℾ, and small raised or lowered gammas (ᵞ, ᵧ) for superscripts and subscripts. The plain γ is still the one to use in nearly every formula.
How to type γ and Γ
On Windows, Alt + 226 on the number pad types the capital Γ. There is no Alt code for small γ, so in Word type 03B3 and press Alt + X, or insert an equation and type \gamma followed by a space.
On a Mac, open the Character Viewer (Control + Command + Space) and search "gamma", or use the Greek keyboard, where G types γ. In LaTeX, use \gamma and \Gamma; in HTML, γ and Γ.
Gamma usually sits next to beta and delta in formulas, and all 24 letters are on the Greek letters page.