Because Gnus treats email too as a form of net news some of the defaults may not be acceptable to people who use Gnus as a mail reader. If you look at the FAQ entry, the suggestion would appear ridiculous that you press a key combination just to see all the messages in a mail group.
A more simpler method (is anything ever simple with Gnus?) is to change the Group Parameters of the group to make it the default behaviour that all mail messages are seen.
In the Group Buffer, against the mail group, hit G P and you'd be presented with a buffer where you need to enter the following elisp form. Delete the nil shown in the buffer and enter
((display . all))
hit C-c C-c
and you're done. Enter the mail group and you should be able to see all the mail messages whether read or unread. Another minor convenience you may like to have is to set it like this.
((visible . t)
(display . all))
which causes the mail group to be visible even if there are no new messages to read. Though for a text based news reader, it will be apparent that this is not exactly convenient as you'd be spending time moving the arrow keys to navigate over mail groups with no new messages.
Rather it's easier to let Gnus tell you that there are unread messages. Unless of course, you spend time rooting through your old mails.
The whole section on reading mails is also quite instructive for other conveniences that you wish you had. At least, this question, is a real time saver and makes things easier on the eyes.
W w is your friend.