Determine the type of MIDI cable you will need. Look at the sound card on your computer to establish what type of audio output port is available. Most sound cards will either have a 15-pin "D" connector or a Universal Serial Bus (USB) port. High-end sound cards typically have an input and output for MIDI cables.
Connect the MIDI output on your computer to the MIDI input on the G-DEC. After obtaining the correct cable for your sound card, connect the male end to the audio output on your computer. Then slide the MIDI OUT portion of the cable into the MIDI IN on the rear panel of the G-DEC amplifier.
Direct MIDI data from your computer to the G-DEC. By default, MIDI data played on your computer will be routed through your speakers. To route it through the G-DEC, first click on "Control Panel" and then click on "Sounds, Speech and Audio Devices." Click on "Sounds and Audio Devices", then select the "Audio" tab. In the "MIDI Music Playback" section, change the default device in the drop-down menu to the device you are running from your sound card to the G-DEC.
Compile MIDI files on your computer. Use an Internet search engine to locate downloadable MIDI files that you would like to play through the G-DEC. MIDI files will have a ".MID" file extension and come in a variety of musical styles. There are also free programs that allow you to create your own downloadable MIDI files using a step sequencer. Create a MIDI folder on your computer and save the files in that location.
Play the MIDI files using a media player. The audio will now be exported through the output on your sound card and played through the G-DEC amplifier's speaker cabinet.
Jam along with the MIDI files. Plug your guitar into the G-DEC's 1/4-inch cable input. Choose a preset guitar tone and play along to the backing MIDI track.