I thought Gulli was reasonable portrayed in "Know No Fear".
In terms of ability, Gulli was head and shoulders above a normal human. He was able to formulate plans--he jury rigged a communication system between his ships on the fly. He also managed to micromanage the mobilization of his Legion, Imperial Army, and Mechanicus units on Calth.
Personality wise he seemed level headed for the first half of the novel, with narration reinforcing it as well.
While pondering whether the Word Bearer's fleet initial attacks were a misunderstanding or a panic response, the Chapter Master of his first Chapter thinks:
"His [Gulliman's] calm is almost terrifying. Gage is transhuman: both bred and trained to know no fear. The acceleration of his own hearts and adrenal levels are simply a response to the situation, a readiness to act faster and more efficiently. But Guilliman is at another level entirely. He is watching a critical disaster unfold on one of his most beloved planets: the miserable loss of a vital shipyard facility, the collateral damage, the destruction of ships, a portion of the fleet crippled, surface locations caught in the debris rain…"
Watching a good portion of his Legion succumb to disaster and Gulli keeps his cool. He only really loses it when he realizes that not only is Logar's Legion attacking his forces deliberately, but it was premeditated to a great degree.
As for his men doubting him, it makes sense.
The book mentions that people, namely the other Legions, consider the UMs a bunch of windup soldiers. Unthinking and inflexible. I think the book is trying to refute that and demonstrate (through various character's thoughts and actions, and through the interesting theoretical/practical conversations they have) that the UMs are bred to be thinkers.
His men don't blindly follow him (like, say, the Word Bearer did with Logar), but ask and question. They want to be kept in the loop. They want to know why and how they're fighting.
