The gate was originally sited across what is now Pembroke Road at the corner of Governors Green. No part of the gate has survived, but the Gatehouse is still in existence.

King Williams Gate (outer)
King Williams Gate
King Williams Gate (inner)
King Williams Gate, on Pembroke Road
Pembroke Road (2005)
Pembroke Road
King Williams Gate House

King Williams Gate, built in 1834, was the last of the gates into Portsmouth town to be built. Prior to that time there had seemed little point in having a major entrance on the east side of town as the land beyond it was largely comprised of marshland, known as the Great Morass, although there had been a small footpath through the fortifications since the time they were constructed.
By the time the gate was built, Southsea was beginning to develop as a town in it's own right. Almost immediately opposite the gate a set of Georgian Terraces (Landport, Jubilee and Belle Vue) had been erected in the early part of the 19th century. A large pond which lay between the gate and the terraces was drained by the Board of Ordance in 1836 as part of further development of the area.
The absence of any significant entrance on the east side of town has played a part in the reconstruction of the last walk that Lord Nelson took on English soil. On Saturday 14th September 1805, Nelson breakfasted at the George Hotel, and, trying to avoid the crowds in front of the hotel, left by the back door into Penny Street. From there he turned into Green Row, now Pembroke Road, which Colin White, in his essay on Nelson's Last Walk, describes as being a 'dead-end'. White uses this conjecture to suppose that Nelson must have either crossed Governors Green or followed the inside wall of the fortifications in order to reach the beach at Spur Redoubt.
This account is thrown into some doubt by Nelson himself who, writing in 1784, describes a periless journey on a runaway horse which took him into Portsmouth by the London (Landport) Gate, through the town and "out at the gate that leads to common" - a gate that could only have been at the end of Pembroke Road. Of course this doesn't completely discredit White's theory - Nelson could still have reached the same part of the beach even if he had used the gate in the fortifications to leave the town.
In the end King Williams Gate survived for a mere 42 years before being pulled down in 1876 along with the majority of the fortifications.
The top image (see left) shows the exterior of King Williams Gate below which is a complementary picture from the inside. The third image is taken from roughly the same place as the second, both of them showing the Gate House, towards the lower right corner, which still exists.