Concept:
Gastric juice is secreted by the stomach and contains the enzyme pepsin, which helps in the digestion of proteins.
Step 1: Identify the function of each option.
• Insulin -- Hormone that regulates blood sugar.
• Lipase -- Digests fats (mainly in the small intestine).
• Pepsin -- Digests proteins in the stomach. \checkmark
• Amylase -- Digests starch (present in saliva and pancreatic juice).
Step 2: Final conclusion.
Hence, the digestive enzyme present in gastric juice is
\[
\boxed{\text{Pepsin}.}
\]