The Spanish letter G can be pronounced in two different ways.
1. When the G precedes A, O, U, or a consonant, it is pronounced like a hard English g. It may also be softened to something approaching 2, below.
|agosto||August||Similar English sound|
2. When the G precedes an E or I, it is pronounced like a Spanish J.
None. Somewhat similar to the Parisian R, or the CH in Scottish word loch.
Also see lesson on hard and soft vowels
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.