The words gallop, galop sound the same but have different meanings and spellings. Why do gallop, galop sound the same even though they are completely different words?
The answer is simple: gallop, galop are homophones of the English language.
To ride a horse at a gallop.
A natural three-beat gait of a horse, faster than a canter, in which all four feet are off the ground at the same time during each stride.
A fast running motion of other quadrupeds.
A ride taken at a gallop.
A lively dance in duple time, popular in the 19th century.
The music for this dance.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").