Conversational speech is filled with slang and colloquial expressions. However, you should avoid slang on the GRE analytical writing assessment. Slang terms and colloquialisms can be confusing to the reader, since these expressions are not universally understood. Even worse, such informal writing may give readers the impression that you are poorly educated or arrogant.
INAPPROPRIATE: He is really into gardening.
CORRECT: He enjoys gardening.
INAPPROPRIATE: She plays a wicked game of tennis.
CORRECT: She excels in tennis.