Filemaker is a stand along workhorse and you don't need Google docs or Excel any more if you design your file correctly or find one someone else has designed correctly.
You may first import your Google doc or excel file into Filemaker and then forget it. Use Filemaker for everything.
First you create an item/price list table which will have your barcode info and three calcualted fields, see below.
Then you add a transaction table that is linked to the item file by the barcode field. The transaction table has a minimum of BarCode, Date, Add (number) and Remove (number) fields.
When you add an item, you create a new record and scan the barcode. Enter the number added into the Add field. When removing an item create a new record and add the number removed in the Remove field.
Now you have a chronological record of what happened (much better than any google doc or excel file).
Add the following calculated fields to your item/price list table:
Total Received = sum(Items::Add)
Total Sold = sum(Items::Remove)
On Hand = Total Received - Total Sold
That's enough to get you sailing in uncharted waters and to raise other questions. Good luck and happy sailing.
...or just open the Inventory file supplied with Filemaker 12 and study that.
File:New Starter Solution Inventory.