Out of all students gail completed the assignment more rapidly. is this sentence correct.

 Sadly, that sentence does not follow the rules of correct grammar. The ideas are almost bunched, as well as words and phrases that have rough edges rather than smoothed together. The sentence in it's correct format without changing the idea of it would look as follows:
 Out of all the students, Gail was able to complete the assignment the most rapid when compared to her peers.
 
Sometimes you have to add or take away a few words to make a sentence sound better and flow better as well. Hope this helped!