You’re misreading Gingrich. What he said was that if the GOP tries to install a completely different nominee, other than either Trump or Cruz, THEN all they have to do is form an alliance and put a stop to it, which they would very likely do.
But if the GOP tries to install Cruz as the nominee, as is likely to happen, you won’t see that Trump/Cruz alliance at all. And you’ll quite possibly see Cruz as the nominee unless Trump has a majority going into the convention.
The problem with your scenario is too many Trump supporters would bolt the party and not vote for Cruz.
Curz would secure the nomination, but have an almost zero chance of winning the presidency. A cold prize indeed.