IF you arrive at 5 PM - you'll be into Seoul by 7 PM. KAL Limo Bus from T2 is now faster - all other buses go to T1 and that adds 20 mins.
If you stay at the Grand Hyatt - I would not go all the way back to T2 to take a bus. It's literally right next door to T1 and again - 20 mins by road from T1 to T2.
Seoul is a very safe city and you will have noting to worry about.