The CC-by 2.5 license allows content which is created under it to be
relicensed under another free license which meets its minimums.
The GFDL meets these minimums, but does not allow content created under
it to be relicensed.
What this means is that the CC-by 2.5 content may be incorporated into a
GFDL item. GFDL content may *not* be incorporated into a CC-by 2.5 item.
That's the "one-way compatible" situation.
Sorry if this was not clear.
Amgine