You CBuffer is an illegal size. Has to be a multiple of 16 bytes (yours is 268, closest legal size is 272).
Also, you're guessing at the location of elements in the CBuffer - they're not tightly packed. You really need to use the shader reflection APIs to query where things actually are in the CBuffer.
If the CBuffer is not entirely made of float4's then nothing will be where you think it is (well, the packing rules are documented, but you won't have that stuff in your head - and the reflection APIs eliminate the need to).
The core of what you're doing there is workable, you're just botching CBuffer setup is all (just explicitly writing in 272 instead of params.sum will get you past the error, but won't make your buffer correct).
The absolute easiest way to plug a GS in is to just use MGFX as usual but inject the geometry shader after the EffectPass is applied but before you draw anything.
That only works on DX11 (GL uses whole program linking so you have to roll an entire raw shader library).