The secret room is revealed in Episode 92 - Iron Golem when Ryan Haywood and the other Achievement Hunters walk into Michael's house and see the trap door, having not been previously there. In that time, both Gavin and Michael were hiding under there. It can be found from Michael's house through his trap door in his room, and from Gavin's house through the trap door in his Tower of Pimps room.
Since then it has barely appeared in the series from then on. It made a small cameo in Mad King Ryan Pt. 1, where Ray looks down from Gavin's Tower of Pimps room into the entrance to the tunnel. It also made an appearance in Episode 107 - Halo Mashup, looking much more modernized.
The tunnel and/or Kerry's room have been shown in brief appearances whenever Plan G is triggered, with the diamond blocks shown for a very short second before being obliterated. Since this had been seen numerous times, it is more than likely that Michael or Gavin rebuild it along with the rest of Achievement City.
The tunnels were constructed in Let's Build - Team Nice Dynamite Secret Room in both parts. The tunnel's walls and ceilings are made of diamond and TNT blocks, with a candle poking out of every second emerald block, whilst the floor is entirely made of gold blocks. The tunnel connects Gavin's and Michael's houses. It was built right next to Plan G (a large room filled with dynamite and TNT blocks). The tunnel also consisted of the Nice Dynamite Victory Room, with signs of Michael's name and Gavin's name. The walls in this room are entirely made of gold, and have one Obsidian block in the event that they jointly-won the Tower of Pimps.
However, the tunnel also has a wooden wall with a door that leads into Kerry's rooms. The floor in Kerry's bedroom is made of red carpet, and his door is encased by glass. Above the door a sign reads "Queerion Lannister". Outside of his bedroom is a waiting-styled room made up of wooden walls and a couch and table. This can be instantly accessible from Michael's house, but one would have to go through the tunnel into Kerry's waiting room if they entered through Gavin's house.