To manage the tours your gallery contains:
1) In the 'Galleries' section of your profile, click the gallery you'd like to add the tours to;
2) Click the 'Manage tours' option:
3) Mark the tours with the tick to add those to the gallery:
4) Click 'Back to Gallery'.
Please note that privacy settings are ignored for the tours you add to the gallery. Any tour added to the gallery will be listed there, even if it is set to be unlisted or has a 'Privacy & White list' tour upgrade applied.