Your SSH-keys are not read directly by GitFTP-Deploy. Instead they are stored in the keychain. Pre-Sierra this was not a problem. But on newer versions of macOS this can cause problems if the keychain is locked. I am currently investigating this, meanwhile here are some workarounds.
Manually add key to keychain
ssh-add -K <key>
(it’s recommended to use absolute path of keys) call the command
ssh-add -A on every startup of macOS.
Or create a SSH configuration
Add the following to the file