To calculate the change in government spending as a share of GDP relative to the previous century, we can follow these steps:

1. Calculate the current government spending as a share of GDP:

   $\text{Current share} = \frac{\text{Government spending}}{\text{GDP}} \times 100$

   Using the values given:

   $\text{Current share} = \frac{36.9}{180} \times 100$

2. Calculate the current share value:

   $\text{Current share} = 20.5\%$

3. Determine the change in government spending as a share of GDP:

   The previous century’s share was 2.1%, so:

   $\text{Change} = \text{Current share} - \text{Previous share} = 20.5\% - 2.1\%$

4. Calculate the difference:

   $\text{Change} = 18.4 \text{ percentage points}$

Thus, the government spending as a share of GDP has changed by 18.4 percentage points relative to the last century.