Can I sell gift cards in fixed amounts, 50, 100 and 200, on my ordering page?

Yes. Make three menu items. Open Menu, use the Add item panel, name the first Gift card 50, set the Price to 50, put it in a category such as Gifts, and tick This is a gift card. Leave Card value blank and the card is worth what they paid. Repeat for 100 and 200. Each one carries a small gift card badge in your menu list and customers add it to the basket like a sandwich. When the order is finished the code is made and sent to the buyer on WhatsApp. The Gift cards, store credit and house accounts tab in the docs has the guide.