H-1B Cap Gap gives you an extension of your OPT until September 30th, if as of April 2nd, you are either on an approved OPT that expires before Oct 1st, or in your 60 day grace period. But there are two distinct scenarios.
Scenario 1: If the employer filed your H-1B petition and it is received by USCIS prior to your post-completion OPT expiration date, you qualify for an extension of your OPT employment authorization.
Scenario 2: If the employer filed your H-1B petition and it is received by USCIS after your post-completion OPT expires but during your 60 day grace period following OPT, your F-1 status and permission to remain in the US are extended but you are not eligible to work until your H-1B is approved.
If the student’s H-1B is not selected, the student will have the standard 60-day grace period from the date of the rejection notice to depart the US, transfer the I-20 to a new program, or apply for a change of status. However, if a denial is based on a discovered status violation, no such grace period exists and the student must leave the US immediately.