The answer is a lot of "it depends." I'll get GPS drops on my Garmin here and there on routes with underpasses, and the lapse is maybe 20 seconds. IIRC it records an assumed straight-line path for the drop, and I'd think it'd just do the math on figuring out speed if I didn't have a wheel speed sensor. I also have the wheel speed sensor installed because I use rollers.
If I had to pay for the sensor separately and didn't have a use for it indoors, I'd probably go without and live with the extremely minor GPS drops.