Given a positive integer $n$, an $n$-gun is a $2n$-mino that is formed by putting a $1 \times n$ grid and an $n \times 1$ grid side by side so that one of the corner unit squares of the first grid is next to one of the corner unit squares of the second grid. Find the minimum possible $k$ such that it is possible to color the infinite planar grid with $k$ colors such that any $n$-gun cannot cover two different squares with the same color.

[i]Itf0501[/i]
To solve the problem, we need to determine the minimum number of colors \( k \) required to color an infinite planar grid such that no \( n \)-gun can cover two squares of the same color. An \( n \)-gun is a \( 2n \)-mino formed by placing a \( 1 \times n \) grid and an \( n \times 1 \) grid side by side, sharing a corner.

1. **Understanding the \( n \)-gun:**
   - An \( n \)-gun consists of \( 2n \) unit squares.
   - It can be visualized as an L-shaped figure with \( n \) squares in one direction and \( n \) squares in the perpendicular direction.

2. **Constructing the Coloring Pattern:**
   - Consider a \((n+1) \times (n+1)\) square with one corner cut off. This shape will be referred to as a "shield."
   - The shield contains \((n+1)^2 - 1\) squares.

3. **Coloring the Shield:**
   - We need to color the shield such that no two squares within the shield that can be covered by an \( n \)-gun share the same color.
   - Since any two squares in the shield can be covered by an \( n \)-gun, each square must have a unique color.
   - Therefore, the shield requires \((n+1)^2 - 1\) distinct colors.

4. **Tiling the Plane:**
   - The infinite plane can be tiled with these shields.
   - Each shield will be colored using the same \((n+1)^2 - 1\) colors.
   - Due to the symmetry and the structure of the shields, no \( n \)-gun can cover two squares of the same color.

5. **Proof of Minimum Colors:**
   - Assume there are fewer than \((n+1)^2 - 1\) colors.
   - Consider the lower-right corner of the shield, which is friendly with all other squares in the shield except the upper-left corner.
   - If there are fewer colors, the lower-right corner must share a color with the upper-left corner, leading to a contradiction.
   - Therefore, at least \((n+1)^2 - 1\) colors are necessary.

6. **Conclusion:**
   - The minimum number of colors required to color the infinite planar grid such that no \( n \)-gun covers two squares of the same color is \( n(n+2) \).

The final answer is \( \boxed{n(n+2)} \).