No doom tracker markers when sealing a gate with an Elder Sign
This would be very rare -- the only time a gate would be placed without a doom token is if it came from a gatebuster. But, theoretically, it could happen -- you seal a gate with an Elder Sign, then a gatebuster opened up another gate, then you used a second Elder Sign to seal it. And you got monster surges all other times. 

The two rulings I could see is either you couldn't use the Elder Sign (because you couldn't fulfill the full requirements, just like if you didn't have enough clues or money to purchase something), or you would ignore that part of the card and simply seal the gate. Our group would probably choose the latter (although you probably would be smarter to not use the Elder Sign at this point).