I understand that you are having some issues in order to download the SlingPlayer Mobile application on your ViewSonic gTablet.
I recommend you to check this link in order to get some information about the compatibility with Android Tablets.
Does SlingPlayer Mobile for Android work on my Android tablet?
If you are not able to install this application in a proper way, I recommend you to contact the Android Market support, through this third-party link.
Regarding the last part of your post, this application is available only at the Android Market Place. The following link will provide you with the steps to follow.