"Relative scale" is why you're failing.
Whenever you're working with hydraulic erosion algorithms, like those in Gaea, going very small or very large in scale creates problems. Anything procedural or algorithmic in Gaea breaks down at anything below 1km, or anything above 64km.
Work in "relative scale". Eyball your small scale details, but keep your "world scale" in balance. Go "proportional" instead of "1:1" with anything "scale"
That said…
At the bottom of your node properties, there's "post processing stack". Select the last node in your graph, and expand those post process settings to reveal a checkbox named "drop to floor". That may help.