By default, when Gnus is unplugged, all outgoing messages (both mail and news) are stored in the draft group “queue” (see Drafts). You can view them there after posting, and edit them at will.
You can control the circumstances under which outgoing mail is queued
(see gnus-agent-queue-mail, see Agent Variables). Outgoing
news is always queued when Gnus is unplugged, and never otherwise.
You can send the messages either from the draft group with the special commands available there, or you can use the J S command in the group buffer to send all the sendable messages in the draft group. Posting news will only work when Gnus is plugged, but you can send mail at any time.
If sending mail while unplugged does not work for you and you worry
about hitting J S by accident when unplugged, you can have Gnus
ask you to confirm your action (see
gnus-agent-prompt-send-queue, see Agent Variables).