"GoAnywhere OpenPGP Studio Installer" is damage and can't be opened. You should move it to the Trash.
This happens whether I try from the one on the mounted disk image file itself, or alternatively try to copy it from the mounted disk image to the desktop before double-clicking on it.
1. Open the System Preferences. This can be done by either clicking on the System Preferences icon in the Dock or by going to Apple Menu > System Preferences.
2. Open the Security & Privacy pane by clicking Security & Privacy.
3. Make sure that the General section of the the Security & Privacy pane is selected. Click the icon labeled Click the lock to prevent further changes.
4. Enter your username and password into the prompt that appears and click Unlock.
5. Under the section labeled Allow applications downloaded from:, select Anywhere. On the prompt that appears, click Allow From Anywhere.
6. Exit System Preferences by clicking the red button in the upper left of the window. You should now be able to install GoAnywhere OpenPGP Studio as well other applications downloaded from the internet.
In fact, it's well enough to toggle the setting in the Preference Pane (as described above) but leave the Preference Pane open; install and launch GAOPGPS; turn the setting back on; and only _then_ close the Preference Pane.