In gastric outlet obstruction in a peptic ulcer patient, the site of obstruction is most likely to be-
Bailey writes - "The term 'pyloric stenosis' is a misnomer. The stenosis is seldom at the pylorus. Commonly, when the condition is due to underlying peptic ulcer disease, the stenosis is found in the first part of the duodenum, the most common site for a peptic ulcer."

Also, know

There are 2 common causes of gastric outlet obstruction
pyloric stenosis secondary to peptic ulcer
gastric cancer
Earlier peptic ulcer was the most common cause, but now the most common cause of gastric outlet obstruction is → gastric cancer.
Antrum
Duodenum
Pylorus
Pyloric canal