For SMARTDAC+ GX/GP series, can I eject the SD memory card during recording? Also, while the SD memory card is being ejected, what happens to the data being recorded?

As long as the unit is not in the middle of writing to the SD memory card, you can eject the card with no problem, even during recording.

Data being recorded while the SD memory card is not inserted is saved to the recorder’s internal memory.

When you put the SD memory card back in, you can perform the “Save unsaved data" operation to copy the unsaved data to the SD memory card. Note that even if you don’t perform that operation explicitly, that unsaved data will also be saved to the SD memory card the next time you do a file save.