Currently, Gather does not have a mobile version in the App Store. You can, however, log into your Gather account via the browser on your phone or tablet.
All you need to do is login using this link: https://go.gatherit.co/users/sign_in and the email and PW you normally login with.
To make this whole process easier, you can actually add a link to the sign in page as a button on your phone or tablet.
If you have any questions, please let us know!