Large SiGe ASICs for digital logic
NPN and PNP transistors use more current when building logic. You need to pass current through the base-emitter junction in order to turn them on. In MOS, to first order, the only current you need is to charge the gate capacitance. 

This is why CMOS has replaced bipolar in almost all logic applications (there are exceptions, such as ECL in PLL dividers). 

So to answer your questions: 


  Are SiGe NPN transistors used to build digital gates?


Yes. This is done either for very simple, small logic, when you might need a few hundred control gates in a power amplifier or similar (and you can save the cost of the BiCMOS part in the process), or when logic needs to be really fast (for example in some of the building blocks in PLLs). 


  Can digital ASICs be built from tens of millions of SiGe gates?


While I don't think there is a physical reason why not, the current required to power such a chip makes it practically unfeasible - it would get very hot and expensive. The advantages of CMOS just make it to be an excellent digital technology.