They may only guarantee the GCs for 60 days, but there is nothing to stop you from combining those funds on a different Target GC, which prevents the original owner from accessing those funds.
I use the Target app to transfer the money from GCs bought on Raise .com all of the time. I use the app on my phone to add the new GC and then I combine it with a GC that I have the physical card for. The GC that I purchased from Raise zero's out and can no longer be used for purchases. It goes into what they call a forced close status.

The only downside is you can only combine up to $1000 or maybe it's cards less than $1000. So buy the smaller cards
