Hi Luo,
1) I meant the native WordPress image gallery for each post. So basically most post gallery plugins automatically use the images uploaded to a post, you don't have to select the posts in the post's media uploader and insert a gallery shortcode. So when someone creates a post via CRED and uploads images the post's image gallery should automatically use that posts. Your reply to 2) should apply here then (I searched the forums but it wasn't exactly clear if CRED can upload/assign images to a post's gallery).
3) OK, thanks! (please do think about adding that to the plugin's core though, could be very useful to other users as well)
Cheers,
Thomas