There is likely a way to filter the Report to remove GC Sales, however, the default Work Period Report is not configured to do so.
I use a SQL Script for my Z Reports which filters out the GC Sales and only shows Product Sales. I don't use the default Work Period Report because I don't like the data it shows, and GC Sales is one such thing.
How you want to slice it is up to you or your local laws. To me, a GC Sale is not a Product, and it has no Tax as such. It is simply a trade of Cash for a Paper Certificate. So the Cash is reported as a Payment or a sort of Income, but only for the purpose of balancing the Cashout Report at the end of the day to reconcile Cash in Drawer. It's like having a Customer give you money to put on their Account as a Credit to be used at a later time. We don't know which Product(s) the Credit (or GC) will be used for, and whether or not they are Taxable Products.
So at a later time, when an actual Product is purchased, Sales figures are reported for that Product (Base price plus applicable Taxes). Any method of Payment (or combination of methods) can be used, be it Cash, CC, GC, Customer Account, etc.
Probably, but that doesn't sound right to me. I would guess you would need to use a Fixed Amount Ticket Calculation for that instead of a Payment Type.
In any case, the way GC's are currently set up as per the Tutorial, they are both a "Product" and an Entity with an associated Account. It could be reconfigured so that the Product does not exist at all, but at the time the Tutorial was written, using a Product was a simple way to get a button on the Menu. Now we can have Custom buttons on the Menu which can do anything we want, effectively invoking the same sort of Automation that we currently use for GCs, but in a way that eliminates the Product itself. That might be worth looking into.
All that said, if you want to continue to use the default Work Period Report in the interim, I would probably look for a way to alter it to filter out certain Transaction Types. For example, the GC explicitly uses a different Tx Type rather than the regular Sales Tx Type. That was exactly the intended purpose - to exclude the GC Purchase from actual Product Sales ...