Sentence fragments on the GMAT usually show up as dependent clauses pretending to convey complete thoughts or as a bunch of words with something that looks like a verb but doesn't act like one.
Dependent clauses standing alone are fragments because they don’t present complete thoughts.
For example, this clause comes complete with a subject and verb: Although many companies have failed to maintain consistent profits with downsizing. However, it begins with a subordinating conjunction, although, so it leaves you hanging.
Phrases with a verbal instead of a verb can appear to be complete if you don’t read them carefully.
The verbal phrases in this sentence look like verbs but don’t function as verbs: The peacefulness of a morning warmed by the summer sun and the verdant pastures humming with the sound of busy bees. Warmed and humming can function as verbs in other instances, but in this sentence, they are part of phrases that provide description but don’t tell what the subjects (peacefulness and pastures) are like or what they are doing.
Correcting fragments is usually pretty simple. You just add the information that completes the thought or change the verbal phrase to an actual verb.