Given specific rewards, how can I calculate the returns for each time step?

Let's use Excercise 3.8 from Sutton, Barto - Introduction to RL:

Suppose $$\gamma = 0.5$$ and following sequence of rewards is received $$R_1=-1$$ , $$R_2=2$$ , $$R_3=6$$ , $$R_4=3$$ , $$R_5=2$$ , with $$T=5$$ . What are $$G_0, G_1, ..., G_5?$$

There isn't $$G_5$$ because $$R_5$$ is last reward. Am I understanding it right?

So:

$$G_4 = 2$$

$$G_3 = 3 + 0.5*2 = 4$$

$$G_2 = 6+0.5*4 = 8$$

$$G_1 = 2+0.5*8 = 6$$

$$G_0 = -1 +0.5*6 = 2$$

To back up your intuition about there not being a $$G_5$$, refer to the definition of discounted return in the periodic case (3.11). $$G_t \doteq \sum_{k=t+1}^T \gamma^{k-t-1} R_k$$
You'll see that $$G_5$$ would be written as a sum with no terms in it, since $$T=5$$.