The total number of $4$-digit numbers whose greatest common divisor with $18$ is $3$ is _____.

Let $N$ be the four digit number $gcd\left(N,18\right)=3$

Hence $N$ is an odd integer which is divisible by $3$ but not by $9$.

$4$ digit odd multiples of $3$

$1005,1011,\dots ,9999\to 1500$

digit odd multiples of $9$

$1017,1035,\dots ,9999\to 500$

Hence number of such $N=1500-500=1000$