Finding electron configuration of lanthanide ions
I agree with bobthechemist's comment.

When you are determining the electronic configuration of anions/cations, you will first need to determine electronic configuration of neutral atom. Like in the case above:

$$\ce{Gd} : \mathrm{[Xe] (6s)^2 (4f)^7 (5d)^1}$$

Now arrange this configuration according to ascending value of $n$:

$$\mathrm{[Xe] (4f)^7 (5d)^1 (6s)^2}$$

Now, according to your need start subtracting electrons from highest value of $n$, like for $\ce{Gd^3+}$, the electronic configuration will be:

$$\ce{Gd^3+} : \mathrm{[Xe] (4f)^7}$$

See this to know more on calculating electronic configuration of cations and anions.