I am trying to duplicate your steps to help you out. I have generated an SP List and created the same App Layout in Power Apps.
You may just need to Refresh your SharePoint List data source. Go to Data - find your SP List - and click Refresh.
Unless you have made changes to the App, your Gallery and the Form to the right in the layout should both be connected to the same SharePoint List.
If you click on the Gallery and select "Items", you should see something similar to:
When you add a new item in the SP List from SharePoint, does that item show up in the Power Apps Gallery? I did a test where I added an new item to the list in SharePoint. If I typed in the "Search" field in the App, the Gallery refreshed. I had given my new item a specific Title. I typed the new Title into the Search field and the Gallery filtered to that item.
If for some reason none of this is working for you, can you check the number of items in the list? You can do this by going to SharePoint, showing the ID column, and sorting the ID column descending. The ID number of the most recent item will often give you an indication of the number of items in the list. If you are below 500, the the number of items in the list is probably not causing an issue. If the number is greater than 500, reply back here.
If none of this helped, reply back here and the forum can try to help you further.
If I've answered your question or solved your problem, please mark this question as answered. This helps others who have the same question find a solution quickly via the forum search. If you liked my response, please consider giving it a Like.
Thanks
-Mark