Yes. The GT-1000CORE's SEND (1, 2) jacks can be used as SUB OUT jacks. Set SEND 1/2 SETTING to "SUB OUT" or "SUB OUT, AUX MUTE" in HARDWARE SETTING.
[MENU] button > HARDWARE SETTING > OTHER
Note: When "SUB OUT" is selected, the RETURN jack functions as an AUX IN jack. If no plug is connected to the RETURN jack, the SUB OUT signal will also be output from the OUTPUT jack.
For details, refer to "HARDWARE SETTING" > "OTHER" in the GT-1000CORE Parameter Guide.