If you wanted to continue using stepper motors, the ZETA drive is available to replace the GT drives.
The ZETA4 is 4Amps and the ZETA8 is 8Amps, both are 120vac inputs.
The Zeta is nice because there's no software configuration and only uses dipswitches on the top to set for the motor.
The Zeta take standard step & direction signals, like the GT drive in DMODE6 or DMODE7 (step & direction) which is the most common mode used on the GT.
The GT also supported DMODE4 (analog velocity) and unfortunately that is not supported on the Zeta.
The other options are to switch the motor to a servo motor and use the Gemini servo (GV) but that's an older product. The better choice if open to upgrading the motor type would be to consider the newer Pseries indexer drive. It supports both analog velocity or step & direction modes. It has auto-tuning and smart detection with the Pseries servo motors.
Servo motors do have advantages of:
1. They're more accurate because they're a closed loop system. Step motors can open loop and can stall/lose position and you don't know it without an encoder; by the time you add an encoder on the back of the motor, you're better off with a servo 99% of the time.
2. They run cooler and are more efficient because they only draw power as needed because it's an error driven system. Thus you're not using very much power when you're not moving; step motors with most drive are running at current when stationary. This makes step motors run hot.
3. Servo motors can run at higher speeds. Step motors top speeds are 50rps (3000rpm) (with very little torque to actually accel/decel the load) if it's a good stepper drive like the Gemini/Zeta. Servo motors can run at 83rps (5000rpm) and higher, increasing your machine throughput and decreasing per part cost.
4. Servo motors are quieter. Step motors are noisy at speeds above 10rps (600rpm).
GT-L5, GT-L8, GT-L5-NK, GT-L8-NK