I've enabled GPG commit signing on the command line, and manually enabled via the "Repository Settings" section of SourceTree for each repo:
Screen Shot 2016-04-11 at 15.12.48.png
If I manually select "Sign commit" each time SourceTree correctly signs my commits:
Screen Shot 2016-04-11 at 15.16.48.png
However any other operation like stash or rebase doesn't seem to include the GPG options in the command and I get the following "gpg: cannot open `/dev/tty': Device not configured" error:
Screen Shot 2016-04-11 at 15.18.41.png
I see you even have a support article about this, but the proposed solution is enabling GPG signing in SourceTree which I've already done.