GPS can be loss if you unlock on the iPad Baseband. To be extremely honest, I haven't used UltraSn0w to unlock anything since iOS 5.0/5.0.1's fairly initial release, and add on to the fact that I've got it working on both an old/new bootrom 3GS and on an iPhone 4 on the 01.59.00 baseband, all of which were successful on the first go. However, based on the prior experience, the reading of articles/discussions on the topic (as well as using several forums as lab rats hehe), it all pretty much built up to the point that "X will not work based on trial and error" and "Y will work based on trial and error". So when I say that the GPS component can be loss on the 06.15.00 unlocked device, it's from reported happenings and such, but, there are fixes for it, and that much has been made apparent through a lot of sources.
With respect to bricking, I've dealt with bricked devices before, and from very minor testing/theorycrafting, iOS 4.1 seems to be the bridge that allows new bootrom 3GS's to flash and downgrade their baseband. From the scenarios that I've dealt with, there hasn't been a case where the 3GS was not successfully flashed and downgraded in terms of its baseband, but in (I would say) 50% or so of the cases, GPS was the underlying issue that aroused after unlocking, but repeating the entire process of getting the latest baseband, downgrading to iOS 4.1, re-flashing and re-downgrading, then restoring, then re-jailbreaking has ended up resolving the issue.
From stories and from cases that I've read, the only time that a user has experienced a bricked device with respect to the baseband in general, is when the user attempts to replace their baseband chipset directly from another device, the user attempts to flash the baseband on a new bootrom 3GS whilst on whatever the latest iOS is, or when the user attempts to downgrade from a baseband that is not 06.15.00. The margin for error in anything hacking/jailbreaking related is relatively small and stepping pass that margin would result in a one-way ticket to restoring your device, which isn't the end of the world for a 3GS, but just saying in general. In your case, the risk of bricking your device is extremely minute assuming you don't undergo the "smash my keyboard and mouse first, and ask questions later" mentality that (unfortunately) does plague a lot of the hacking world.
TL;DR: GPS can be recovered if loss, device will brick if and only if you close your eyes while a toddler is smashing keys on your computer and not ask questions if you have any.
