My Mac displays some glitches
If the Genius said that the problem is in the cable, you should try swapping the cable.

With that said, TEST YOUR GPU NOW. If there's indeed a problem, you really want to get it fixed now.

Run Apple Hardware Test (if you computer was released before June 2013) or Apple Diagnostics. Make sure that you run the relevant tests. Run some sort of GPU stress test like Unigine Heaven. If anything seems to be triggering errors, bring it to the Apple Store and try to get them to replace it. GPU replacement is expensive and you'd be much better off having Apple pay for it than you.