That glitch you see is being caused by a conflict with some other software on your site interfering with the form builder. I’d recommend fixing that rather than working around it by trying to figure out the merge tags.
You can begin troubleshooting by checking for theme and plugins conflicts.
For single WordPress installations you can use the Debug add-on. It will allow you to perform the test easily disabling things only for your logged in user only, not for your visitors. Make sure that you have a WP default theme without custom functions installed in the site (e.g. a fresh copy of Twenty Twenty) before enabling the add-on, no need to activate it, just install it. Detailed instructions on how to use this add-on can be found here: https://docs.gravityforms.com/gravity-forms-debug-add-on/
If you’re running a multi-site WordPress installation, you will need to perform the testing manually following these instructions: https://docs.gravityforms.com/testing-for-a-themeplugin-conflict/
Let us know what you find out.
If you really want to create the merge tags yourself, it’s not too hard. You can use only the field ID. So if you had a single line text field in the form, field ID 7, which had the label of “Name of School” (for example) this merge tag would work for you:
It gets trickier with complex inputs like Name and Address, and checkboxes, which is why I recommend fixing the problem rather than trying to work around it.