Can you provide more context?
GCSR controls the peripheral clock. I'm not sure what the power up default is, but it probably isn't important since the BIOS is going to change it before your program starts.
You can look at the BIOS source to see how it sets GCSR, or likely read GCSR or GCSRShadow in your program to see the current setting.
From my quick read, it looks like it might default to using the main oscillator clock and be identical to the CPU clock.
Does that answer your question? How does knowing the default frequency affect how you're going to write your code?