Is it possible to get married to a Green card holder while visiting the US on a tourist Visa, and file for adjustment of status I 485 and continue to stay in the US beyond 6 months?
It is not recommended to enter the US on visitor (B1/B2) visa and then marry and change your status without ever going out of US.
This is against the B1/b2 visa rules as you are violating your ‘INTENT’.
You can off-course do marry and change the status within US, but you will have hard time explaining your INTENT to the immigration officer later at the time of permanent residency (Green card).
People, who can convince, do get the Green card but it is difficult and you would be complicating your case if you had entered US with visitor intent.
We advise you to move out of US, and then re-enter with the legal process and valid visa after your marriage with US citizen.
Visitors are required to possess non-immigrant intent (i.e., a residence abroad that they have no intention of abandoning) at the time of entry.