I submitted my GPG key to the keyserver earlier. Now I got signatures on my key. Can I republish it?
gpg --send-keys doesn't seem to alter my key any more, but doesn't spit out any error either.
Yes, of course. Sending should update the copy on the keyservers adding the new signatures (note that it is possible that they already had those signatures which would lead to no changes on their copy).
Additionally, while this would be the traditional action by the keyservers, please note that some keyservers may be read-only (not allowing you to upload keys by hkp), or even discard signatures automatically if they are from unknown keys.