Currently MaxVector supports SVE well in VectorAPI, which is runtime detected.
The size is hardware implementation defined multiple of 128 bits, up to maximum of 2048 bits.

In current gather/scatter API implementation, there's an "int" vector used as index map.
Therefore for IntMaxVector/FloatMaxVector, index map could be vectorized by the same Vector size.
However, for DoubleMaxVecot/LongMaxVector, index map's size is half of vector size.
E.g. For a SVE maximum vector size of 384, the index map's size for Double/LongMaxVector should be 192.
But there's no existing species of vector for 192 bits.

I would suggest to introduce mask to represent this.
In Double/LongMaxVector's gather/scatter API, we always use IntMaxSpecies for indexMap vectorization and half data is masked.