All that we can see Ian enter of the final code appears to be 969, which is missing either the first, or last digit.
We then see Ramsay (in the control center) enter 9517 on his terminal, after which we see the gate unlock.
So the way I see it, Ian never actually guessed the code, and Ramsay simply remotely opened the gate for them.
Now as for what Ian might have tried, my guess is 1969. It's the year of multiple things Ian might be interested in, such as the original Woodstock (his second guess was Miles Davis' birthday), the Apollo 11 moon landing, etc. But also, in Jurassic Park 2, Ian's car (the one they use to rescue the baby T-Rex in San Diego) is a 1969 Pontiac Custom S.
I don't know what would be the significance of the actual code (9517), and there simply might not be any, a reflection of how Ian was trying to use personally meaningful dates as codes, while in real life, such a gate's code would simply be something random.