These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.99 s (backtrack: 0/20).

[ebuild  N     ] sci-mathematics/gmp-ecm-7.0.6-r1::gentoo  USE="custom-tune openmp" CPU_FLAGS_X86="sse2" 949 KiB
[ebuild  N     ] sci-mathematics/msieve-1.53-r1::gentoo  USE="ecm zlib -mpi" 954 KiB

Total: 2 packages (2 new), Size of downloads: 1903 KiB