You have entered into a contract of employment (offer of employment as PGET) with mutually agreed terms and conditions. The terms of the contract of offer, even though you worked for a single day, bind you. Accordingly, you need to pay one month's stipend to the employer in lieu of one month's notice.
Please appreciate the time and efforts the employer put into selecting and offering you the position as PGET. Your decision to join and then leave so abruptly could be seen as a form of disrespect, and the employer may rightfully claim one month's stipend from you.
Therefore, it is advisable to make a thoughtful decision before joining an organization, especially when you have multiple offers at hand or anticipate receiving them within a short period.
I suggest you appeal to the HR department to relieve you, requesting a waiver of the notice period pay (stipend) with an apologetic tone. This approach may lead to a positive outcome.
Regards