How to tell Gigolo / GVFS to use SMBv2 for Windows shares?

Yes, but it comes at a price. Unlike mount.cifs gvfs and thunar use libsmbclient to access shares and it limits itself to the NT1 (SMB1) dialect.

But you can change that by editing /etc/samba/smb.conf and adding the following to the [global] section:

client max protocol = SMB3

There's another one at the minimum but it normally isn't required to do anything with that:

client min protocol = SMB2

The server and the client will auto-negotiate the best dialect to use between those two values.

The problem is this will disable host browsing so if you go to Network in your file browser it will be empty. Accessing it by name still works but must be done explicitly.