You have Game King 4.0. You need Key 17 to key the machine. Check with member RB here, he can fix you up with it.
You may be able to add additional games to what you have available in your menu selection now, but you can't add any new games that are not part of the software set.
You can change the software out on your MPU and play different games like Texas Tea, but you lose the poker games when you install the Texas Tea software. You may need a different sound board too to play I-games like Texas Tea. GK4.0 uses EPROM based sound software, later stuff uses SIMM based software and requires a multimedia lite 2 board (MML2).
Your current software set has the following games available and you can select 12 games per denomination.
Bonus Poker
Deuces Wild Poker
Double Bonus Poker
Jacks or Better Poker
Joker Poker
ACE$ Bonus Poker
Bonus Poker Deluxe
Double Double Bonus Poker
Deuces Wild Bonus Poker
Double Deuce Poker
Loose Deuce Poker
Triple Bonus Poker Plus
ShockWave Poker
Keno
4-Card Keno
Super Keno
Caveman Keno
Triple Diamond Keno
Top-Bottom Keno
3-6-9 Way Keno
2-4-6 Way Keno
Super Way Keno
Black Rhino Slot
Super 8 Line Slot
Diamond Mine Slot
Double Bucks Slot
On The Prowl Slot
Wild Flower Slot
Lion Fish Slot
Super 8 Race Slot
Blackjack