Downsizing Existing Partition

Hello,

I have a quick question that I could not seem to find an answer for on the forums.

When decreasing the size of a partition, in this case, a primary partition with XP on it, will all the data on that partition automatically be deleted using GParted?  Or is there a chance the data will be saved?

Sorry if this question seems simple, I am not very familiar with partitioning hard drives.