I stumbled over the solution to this one myself.
I assumed that the gpg2 commandline parsing would be intelligent enough to process gpg-specific 'options' and 'commands' in any order, but it seems that is not the case.
In the general introductory blurb of the gpg2 man page, I found:
Code:
Please remember that option as well as command parsing stops as soon as a non-option is encountered
'--output' is an option, and '--export-secret-keys' is a command, so the correct terminal command should have been:
Code:
gpg2 --output somewhere.secret.gpg --export-secret-keys somewhere@gmail.com
Doh!
In my defence, though, the error message was misleading in suggesting that '--output' is not an option - it would have been more accurate to report that options are not processed after commands.