Can a gynecologist detect the use of plan B

When you tell them, they will know.
You would not suggest that you don't inform your doctor of all of your medical issues, medications you have taken. The only way they can treat you properly is to be open and honest with your doctor

No, they wouldn't be able to know you used it, but as Fr_Chuck said, it is important to be honest regarding any health issues. There really would be no reason not to mention the use of Plan B.