How can I fix the GPS problem that occured after jailbreaking iPhone 3GS 4.2.1? I have an iPhone 3GS (old bootrom) and updated to 4.2.1 via iTunes a few days ago. Then I jailbreaked it via redsn0w 0.9.6b6. I only marked the option to install Cydia, nothing else (i.e. I didn't updated the baseband to 6.15.00; in settings on the iPhone, it says I have 5.15.04).

Now, it seems that GPS is not working anymore. Why? How can I fix it?

If there is not really a way to fix it, what are the options? In any case, I must have GPS working and I must have the jailbreak.

Downgrade to 4.2 somehow? Or reset it somehow?
You'll need a copy of your old SHSH blobs. If you don't have it, I don't believe you will be able to downgrade.

If you have your old blobs, grab a copy of the older firmware and follow various guides