how to constraint the gated clock in the Timequest？

If a and b are uncertain, which means the clock waveform at the output of c is uncertain, they TimeQuest (or Classic) won't be able to analyze this design.

For TimeQuest to be effective, you need to properly define the clock waveform characteristics. Even if you define two clocks in input a and b, TimeQuest will simply analyze a as if b does not exist, and b as if a does not exist, but it won't analyze the generated clock in c. In other words, without the proper clock waveform in c, TimeQuest will not do a proper setup and hold analysis.