Assertion (A): In gaseous state $3°$ - amines are more basic than $1°$ and $2°$ - amines. 

Reason (R): In gas phase basic strength depends only on inductive effect.

The basicity of amines depends on the ease of the nitrogen atom to donate its lone pair of electrons.

For gaseous state,

Basicity of amines $\alpha$ Inductive effect

Order of inductive effect is,

$3°$ amines $>2°$ amines $>1°$ amines

Therefore, order of basicity is,

$3°$ amines $>2°$ amines $>1°$ amines

Thus, both assertion and reason are true and reason is the correct explanation of assertion.

Hence, option (A) is correct.