OBJECT BASED – Z GRADIENT MASK V2
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0-1 Gradient from Bottom to Top based on the Object Bounds. If you want to use this with mesh particles, just switch out the 'ObjectPivotPoint' node with a 'ParticlePositionWS' node.
V2: Might be cheaper to do it this way rather than the method shown here: Object Based GradientMask
Thanks to TharleVFX