Whitney's Miltank used stomp on my gastly and it worked.
Is this some kind of glitch.
She did not use Odor Sleuth or that kind of move.
Please tell me why this is happening
No, this is Miltank's ability, Scrappy ( http://pokemondb.net/ability/scrappy ).
"Enables moves to hit Ghost-type Pokémon."
So basically, any pokemon with Scrappy can hit a Ghost type with a Normal-type move. Does that make sense?
Hoped this helped! :D
Did the pokemon use foresight? If one pokemon used that move then the whole team would gain the effect. Also why would this be a glitch? Gliches usually freeze the game or have strange sounds at the same time.