Unique greatest common divisor?

If Gerald immediately knows the pair of numbers based on the GCD that he was given, then the number that he was given must have exactly $2$ single digit multiples. The reasons for this are:

1. When the one-digit number has only $2$ one-digit multiples, then these two numbers will be the only one-digit pair with a GCD of the original one-digit number.
2. If the number has less than $2$ one-digit multiples, then their will be no pair of two one-digit numbers that are divisible by the original number.
3. If the number has more than $2$ one-digit multiples, there will obviously be multiple pairs of one-digit numbers that have a GCD of the original one-digit numbers.

We can easily deduce that, as explained by @Saksham Jain and based upon the above rules, the one-digit GCD cannot greater than 4, as these numbers (greater than 4) have only $1$ one-digit multiple (themselves). In addition, we can see that the one-digit numbers $1, 2, \text{and } 3$ cannot be the GCD, as each of these numbers have more than $2$ one-digit multiples.

We see that $4$ is the only one-digit number with exactly $2$ one-digit multiples, making $\boxed{4}$ the one-digit GCD that was given to Gerald.

IF HE IMMEDIATELY KNEW THEN THERE ARE ONLY 2 SINGLE DIGIT MULTIPLES OF IT.THEREFORE ANSWER IS 4 AND NOS. ARE 4 &8 if 2 is correct then nos.can be 2,4,6,8,similary 3,6,9 can be nos. if answer is 3. $$ the answer cannot be larger than 4 or smaller than 2 because there are 2 single digit numbers, if GCD is 1 then nos.can be any pair like 2,5,etc. and it cannot be larger than 4 as if it is larger then there cannot be 2 single digit nos.