The relationship between the specific heat capacities at constant pressure (\( C_p \)) and constant volume (\( C_v \)) is given by the equation:
\[
C_p - C_v = R
\]
Where \( R \) is the gas constant. Substituting the given values:
\[
R = C_p - C_v = 29 \, {J/mol·K} - 21 \, {J/mol·K} = 8 \, {J/mol·K}
\]
Therefore, the gas constant \( R \) is \( 8 \, {J/mol·K} \).