Gonu has 10 minute winds of 115kts and a estimated central pressure of 930 hPa [IMD Weather Synosis]. That means it is a Very Severe Tropical Cyclone on the N Indian Ocean Cyclone Scale.

This is indeed the first Category 5 Cyclone in the Arabian Sea. These are all the other Major Cyclones in the Arabian Sea which only reached Category 3 Status:
1977 at 110 kt
1998 at 105 kt
1999 at 110 kt
2001 at 110 kt

Dry air will be a problem as it moves closer to the coast of Oman and will probably weaken it slightly. The JTWC has Gonu landfalling as a 125kt [140mph] Category 4 Cyclone.