The Gateway is what Myotismon and the kids used to get to Earth. With the help of 9 cards and his power Myotismon opened this gate. Although on the dub side of the show they said In-Training, Rookie, Champion, Data Virus and vaccine the true gate set up is this.
Rookie, Champion and Ultimate. The 'top' of the gate should be Vaccine, Data, Virus. The cards were laid out in that order on the gate.
When Tai used the cards he had to figure the final cards as there were 10 he chose the Gomamon one and got it right but we know in Adventure 02 that Oikawa/Myotismon used Agumon in that place so my guess is that to go from Earth - Digiworld this was right.
*note picture is from the Tokyopop Manga that i scanned*