Is possible to build -168 dBm amplifer?
Sure, you can amplify anything you want. In fact, any amplifier is just fine with a -168 dBm input, provided 168 dBm isn't over its maximum input power specification (which I guess is true of just about every amplifier). If you want a -168 dBm input to become -90 dBm, then you need a 78 dB amplifier. That's nothing special.

However, that won't do anything to improve your situation. Every GPS receiver already has an amplifier. The problem is, as you mention, that the SNR is unchanged at best. In practice, an amplifier has active electronics that adds additional noise. This additional noise is usually specified as a noise figure. It isn't the low power of the GPS signal that's problematic, it's that the signal-to-noise ratio is insufficient to reliably extract the signal from the noise. Amplification doesn't change that.