I'm getting a new computer, it has onboard sound which is pretty good, digital outputs, but no digital inputs

BUT the boss GT8 has midi input/outputs and you can get USB to MIDI cables to control with your computer.....

SO can I record music through these MIDI cables?
I don't know the GT8, but I doubt it. On these multifx boxes, the midi port is used for controlling the device itself, or controlling an external device. It could pass midi triggers back and forth, but it's not going to pass an actual audio signal through the midi cable.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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Yeah. I should of thought about this before seeing as I know MIDI only sends digital messages.......

Anyway I just read the manual and yeah, the MIDI in and outputs are just for controlling it and selecting different settings etc.

No, MIDI to USB is only for patches etc.. As you said
If you want to record you just have to put the gt-8 direct into you soundcard with a cable that fits both