GUM-MNL is actually approximately 13 times weekly on UA (1-2x daily nonstop plus 2x weekly via ROR or TKK). Philippine Airlines (PR) also flies the route daily, and Cebu Pacific (5J) flies it 2-3x weekly. But MNL isn't a great airport for connections, and I don't see any Indian destinations ex-MNL at all. Also, I don't know your nationality, but the Philippines would require a transit visa -- and, according to TIMATIC, an airline escort (!) -- for Indian nationals changing terminals, which you would have to do if you were transiting from UA to ANA, Cathay Pacific, Singapore, Philippine Airlines, or Cebu Pacific.
The bigger problem trying to get to India from Guam is the flight times. Most UA flights ex-GUM are evening departures, timed for connections from HNL. So, you end up with a forced overnight at MNL, HKG, or wherever.
To answer the original question, though -- yes, I would allocate a fairly large buffer in GUM, especially if you're connecting onward on a separate ticket. It's not that the likelihood of a mechanical issue is any higher than any other flight -- it's that if there's a cancellation, the next scheduled flight from that airport might not be for another couple of days.
Hope this helps.
