What is the impact of fiscal stimulus on GDP and price level?

The answer to this will depend to a large extent on where
the economy is on the aggregate supply curve.

If the
economy is in the Keynesian range of the AS curve, the fiscal policy will increase GDP
without increasing the price level.  This is because, in that range, the AS curve is
flat.

If the economy is in the intermediate range of the AS
curve, the fiscal stimulus will increase both GDP and the price level.  The AS curve is
upward slanting in this range.

If the economy is in the
classical range of the AS curve, a fiscal stimulus will result in no increase in GDP but
it will raise the price level.  In this range, the AS curve is
vertical.

So, the impact that a policy of fiscal stimulus
will have on GDP and price level depends mostly on where the economy is on the AS
curve.