I think is is maybe what you're after, in the Sparkle Xcode project:
- Select the project itself to show the various target settings etc
- Select Sparkle under PROJECT
- Select the Info group (from the "tab bar" along the top of the pane
- Under Configurations delete the third one, release dual mode
This removes the configuration which supports Sparkle dual-mode with GC and should remove the warnings. The standard Release configuration does not use GC.
HTH