Unfortunately, we do not have documentation on the GATT database. Please refer to the code in wiced_bt_gatt_db.c in snip/bluetooth/ble_proximity_reporter/ for building up a GATT database.
The native ble_proximity_reporter app does not use notifications however. The following blog will show you how to add the notification feature to the ble_proximity_reporter app.
Also, if you would like to use user-defined strings please refer to the following post.
I am not sure about the Object Transfer service. Perhaps btta would have some information on this.
I hope this helps!
I don't think we support ots yet. But you may wish to refer to "Long Characteristic Big MTU" app in the our wicked smart space.