A hemispherical shell is placed such that its centre is on the positive x-axis. The gravitational intensity at the centre of hemispherical shell is directed along.

If we consider a full shell, the field inside the shell at any point will be zero.

The effect of one hemisphere will be cancelled out by the opposite.

So the hemispherical shell will have a gravitational intensity directed towards the shell from the centre of the hemisphere, here that is along the negative x-axis