OK While doing the Gambit quest for the NCR,I came across "Nothin' but a hound dog" quest with the Kings at the same time. The King says something like "My eyes on the street tell me you haven't spoken to our mutual friend Orris yet, until you do, we got nothin' to talk about". The GI Blues quest was marked as finished when I sent Pacer to his death by the Van Graff Family. According to Fallout Wiki the quest is broken and I won't be able to finish it. I've installed the last 1.2.0.314 PC patch thinking it would fix this quest but no, it did not. I'm sure some of you GECK wizz know a way around this so I can continue this quest to get Rex a brain. Your help is appreciated.

try going to jacobstown ( far left in mountains) and talk to doctor there

While that will allow the OP to give Rex a new brain, The King will never acknowledge the fact due to being stuck at the point where the OP is supposed to hire Orris and find out why he's so successful at his bodyguard job.

The problem is that killing Pacer during the firefight with the NCR at the end of GI Blues terminates the quest regardless of the circumstances thereof, so when Pacer died during King's Gambit it completed GI Blues as well. Since the OP was apparently still at the point in GI Blues where he is supposed to be investigating Orris he never had the subsequent dialogue with The King, which prevented The King's dialogue options from updating to reflect that.

As I always make sure to complete GI Blues before starting King's Gambit I have never run into this issue myself, but to my knowledge there is no way to alter the condition checks for Pacer dying such that this conflict would be avoided if both are running when Pacer dies during the latter. Then again I am not an expert dialogue modder, so there might be something I am missing. The only fix I know of that will work for sure is to reload a save prior to getting Pacer killed and finishing GI Blues by convincing the NCR troops that The King didn't know what Pacer was pulling; that will keep Pacer alive so that the OP can subsequently sell him out if he still wants to.