Are there any development plans for supporting SHA3 or BLAKE?
These algorithms are already included in the Libgcrypt library. It is only a matter of getting them incorporated into the OpenPGP standard for GnuPG to make use of them.
The GnuPG developers are working towards getting the OpenPGP Message Format updated from RFC 4880. The current work-in-progress ...
It hasn’t yet been officially introduced into the OpenPGP standard, and for that reason GnuPG doesn’t support it
I think this is pretty clear: GnuPG implement OpenPGP. SHA-3 is not in the OpenPGP standard.
...and GnuPG will support it as soon as it does
This just means: As soon as it is in the OpenPGP standard, GnuPG will implement it too.