cannot completely download gmod (possible corrupt files maybe missing executable)

[telgou]

so whenever i try to download gmod it always stop at 80% and sometimes it will resume to download but fails after a secs or mins and when it reaches ~95% it will resume but fails after secs and at 97% it stops immediately as i resume it
whenever i manually try to set on top of the queue or press 'play' it seems to start re downloading but fails and drop out of the queue. i read many community discussions and many of them are having this problem

anything else except gmod downloads normally,
-i tried to reinstall steam but always the same problem
-i tried re downloading from scratch(stopped at 80% and 90%)
-i tried to change download directory
-i tried to change download region
-i verified game cache but nothing
-i deleted the app cache and restarted steam..

*when the error appears and i press "see the steam support site for more information" it sends me to "Failed to Start Game (Missing Executable)" troubleshooting
*i can download everything else except it
*verifying game cache may sometimes make it delete the game and re download it again
*it also drops out of queue when i press 'play' or move it to the top
*after dropping out of queue it shows this : "an error occured while updating garry's mod ()"

-i created 4 steam support tickets since 2 weeks but they are so slowww and it is always the same person responding to me, it is that 'chloé' i fell like he is a bot he always suggests the same thing over and over again excepting things to change
-i looked at this steam discussion thread but no luck : [url]http://steamcommunity.com/app/4000/discussions/1/864975225108152447/#p3[/url]