The Google+ Photos team will be offering G+ Photos merchandise to users who will be uploading high quality photos on Google+.
The Gift Bags will be given to users who will share their cover photos with the Google+ Photos team. The team will select best cover photo every week and make it as Cover Photo of the Week. The bag will have t-shirt, camera strap, and stickers.
Google+ will be initially sending out gifts to every user who they have selected and will later continue sending out gift bags to selected users who have shared photos on the Google+ Photos page.
At last, the "photowalk leaders" will be given Google+ Photos gift bags in the coming weeks. There's not much information on this available as of now.