No, they didn't. Read the codex again.
They theorised how it works, based on its effects. They have not managed to replicate it, and can't figure out the math to make it work. They don't even have a working model of a gauss weapon to retroengineer.
Once I get home i'd be happy to provide quotes to this effect from the codex, but I don't have it on me at work.
They do study and use Alien tech. That's undeniable.
Automatically Appended Next Post: Also to completely retroengineer something, as you claimed above, would mean being able to replicate it in it's entirely, including being able to fire and power it.
They would love to be able to do this, but to date they haven't.
I confidently expect that the next Marine codex after the Necron release will have Gauss Cannon added to the armoury, but until Ward does this the current fluff is that they haven't cracked it yet.
Automatically Appended Next Post: Quotes from the Dex:
In regards to the postulations (theoryhammer

) put forward by Magos Barrous concerning the armament carried by Necrons known as Gauss weapons the following report examines the heretical impossibility (they don't know how it works) of such weapons' operation'
The diagram lists the beam focussing array as 'unknown method' and the Electromagnetic field coils as 'postulated' (they're guessing)
'The design of a gauss weapon with all its parameters kept dynamic to achieve maximum efficiency is a mathematical impossibilty' (they can't do the math

)
'Extensive study of gauss weapons....has ,thus far,been hampered by the lack of a working device to examine..'
Automatically Appended Next Post: The discussion on a weapon impact punching clean through a landraider without deflection of the beam states that they couldn't do it without a power source on the same order as titan/starship weaponry.
Nowhere does it say they have a retroengineered one, working or not.