@Mellifluy
Are you claiming the guidelines changed and we are not allowed to tip on the GC, or are you saying you cannot figure out how to do it?
I have always tipped with the GC while paying with mobile pay. I'm not willing to give my GC info to the server just in case they want to keep the info for themselves. 99% of the time, it is probably fine, but there will that one jerk that will try and keep it for themselves.
There is a small chain here in AZ (not a shop) where they don't allow you to tip with GC UNLESS you specify to the server how much to run it for upfront. If they just run the card for total, they don't allow you to tip. So you have to tell them how much to run the card for in order to tip with the GC.
I also found Texas De Brazil does not allow you tip with the costco GC. It's pretty lame they do not let you do that, but I have seen that happen a couple times at different locations. I would think unless it is in writing, they cannot do that, but most people do not fight it.