In which way I could open NEF files on my ipad?

Do you have iPhoto installed on your iPad? If you import RAW files using the camera connection kit or transfer them from your Mac, iPhoto will display the NEF files, you can even edit them in a way. Only, all these operations are based on the embedded JPEG previews in the RAW file. The iPad does not actually develope and process the RAW files,

see: http://help.apple.com/iphoto/iphone/2.0/?handbuch#blnka7d353af

Note: If you import RAW photos into your device, you can view them in iPhoto, but only the embedded JPEG file in each RAW file is displayed. The resolution of the embedded JPEG file depends on your camera. In iPhoto, RAW-format photos have a RAW label.

I don't use Google Drive. But if you can manage to save photos from your Google Drive to your Camera Roll, they will show in iPhoto.