Step 1: Legal provision.
The minimum voting age for local bodies, including Gram Panchayats, is 18 years as per the Representation of the People Act.
Step 2: Purpose.
This ensures youth participation in democratic processes at the grassroots level.
Step 3: Conclude.
\[
\boxed{\text{18 years (Option B) is the minimum age for Gram Panchayat voting.}}
\]