If you would like to run a Gallery that has submissions that are added by you, rather than by users, you can upload submissions through the backend!
Start by setting up your form with all of the info that you want to be shown on the front end and make sure that those fields are properly connected. Here is an article on how to create a Gallery if you are unfamiliar with how to do so. If you have multiple submissions that you would like to add, make sure that on Step 1 of the form you have changed the "Each visitor may enter" setting to match how many submissions you need to add.
Then, on the Setup page in the entry form section, you'll find a button near the top of the form labeled "Submit an Entry" that you can click on to be taken to the form to enter.
Once you have added all of the submissions, you'll need to approve the submissions to show up on the front end of your Gallery. Here is an article on how to manage Gallery submissions if you need guidance on how to approve your submissions.