1. A memory system, comprising: a memory storage including a plurality of memory blocks, and a controller suitable for selecting a victim memory block, among the plurality of memory blocks, for garbage collection (GC) by: measuring a write amplification (WA) value of a first, current window; comparing the WA value for the first window with a previous WA value for a previous window; calculating and setting a value of a ratio threshold based on the comparison of the WA value for the current window to the WA value of the previous window; and using the ratio threshold to determine which of a first candidate memory block and a second candidate memory block, from among the plurality of memory blocks, to select as the victim memory block.