They have no obligation to fix bugs, so, you'll have to do your best to provide as much information as you can.. verbose/debug pcidump/lspci output for the device, links to datasheets/specifications if they exist and you can find them.
Indeed, you can use
sendbug(1) on another system to assemble and send your bug report.. if the system doesn't have mail configured properly, you'll need to provide another address for them to respond.
It is technically possible to send mail to gnats@ manually, but it should be in the format produced by sendbug.. and you need to be sure your mailer won't mangle lines or anything, no need for a subject on the message as the server will generate one itself.