So, I have seen a lot of posts about people getting locked out of the strip recently, I have experienced this bug myself, and have found a way to fix this.

1 - Firstly, you may need some NCR armor to sneak onto a monorail. Go to Camp McCarran and use the monorail service there.
2 - Secondly, I have heard you can kill a robot outside of the strip gate. There should be five, if you kill any one of them you should obtain a key for the strip gate.
3 - Lastly, for pc users, I heard that you can use the console followed by an unlock command, I'm not sure what the exact command is since I don't use a pc.

Thanks for having a look and I hope this has solved your problem with the strip gates.

EDIT: For the third point, I have been told that you would open the console, click on the door and type 'Unlock'.

Bring down the console, click on the door, then type unlock.