Necati,
The Gift Certificate app doesn't provide the option for customers to set their own pricing - that would be this extension: https://www.j2commerce.com/extensions/apps/donation (which is used for a different purpose altogether).
You can configure different amounts as either regular options on a simple product page or as variants using flexvariable or variable product types (which is what I would use).
The instructions found here: https://docs.j2commerce.com/v6/catalog/creating-options can help you get started on configuring the Amounts (which is what I would name the new option in English) and then create new prices such as 25, 50, 100, 200, 250, 500 (or whatever values you wish to offer.
Once you have created your product options, then follow these instructions for the Variable product type to auto create the new gift card amounts so you can set their own skus, pricing, inventory (if desired) , etc.
https://docs.j2commerce.com/v6/catalog/product-types/variable_product#creating-variants
After you have set that up, you can even create different card images with their values showing on the cards and then assign each image to their respective variants so when they click on 50, it would show the gift card with 50 showing, etc (optional)
You can set the global settings for all gift cards in the App's settings page - and you can also set product specific settings that would override the global settings in the Apps tab of the product's edit form view.
If you have any other questions - please don't hesitate to reach out.
Adam