One of the Amazon reviews of the GNS 5870 says the seller is falsely claiming the gps is certified by Apple. I don't know either way. The reviews are not that great either.
There is a much cheaper Bluetooth GPS on Amazon. [ame="http://www.amazon.com/iBlue737A-i-Blue-737A-Bluetooth-Receiver/dp/B000KH7MZ4/ref=cm_cr_pr_product_top"]Amazon.com: iBlue737A+: i-Blue 737A+ Bluetooth GPS Receiver (66 ch, AGPS, Auto On/Off, WAAS): Electronics[/ame] $62, sold by Amazon and eligible for Prime/free shipping. Since it's sold direct by Amazon you have the excellent Amazon return policy to protect you if you're not happy with it for any reason. The GNS is 3rd party, so you're dependent on their return policy - which is unlikely to be as generous as Amazon's.
I haven't used the 737a, so I have no idea how well it plays with GPS apps. Again, the Amazon return policy would protect you if there are any problems. The reviews for it on Amazon are excellent.