Witnesses said the water was crowded at the time of the accident and that the waves were big, but the break zone was shallow.
They reported seeing Gilhool riding a wave, but his board hit another surfer who was hidden beneath in the water, throwing him off.
Another rider who was nearby told police the wipeout didn't seem particularly bad, the Ventura County Star reported; however Gilhool had severely impacted his head, knocking him unconscious. Fellow surfers paddled him to the shore on his board, where they attempted to render first aid.
As first responders summoned an air ambulance, Gilhool went into cardiac arrest. The helicopter arrived and hovered above the beach, but never set down. Instead, the patient was rushed by ambulance to Community Memorial Hospital, but was later pronounced dead. The official cause of death has yet to be revealed.
The second surfer, who according to witnesses had his head and nose sliced open by the fin of Gilhool's board, managed to drive himself to hospital.
Gilhool was the tour manager of the band Dawes, whose lead singer Taylor Goldsmith is married to Mandy Moore.
As Mandy's heartbroken tribute revealed, he was a very close fiend of the couple's.
"Oh goodness. It's so hard to write these words," she wrote, sharing several pictures of them together. "Yesterday, our dear friend Gerry G passed away suddenly in a surfing accident and it just doesn't feel real."
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"Gerry was a towering presence and personality... and if you were lucky enough to be in his orbit and you were a friend, his loyalty and love knew no bounds. I spent many a @dawestheband show with him, side stage, dancing and laughing and confiding in him how much I was falling in love with my now husband. Some of my fondest memories."
"I've never known someone to take as much pleasure in helping others as Gerry. I've also never know someone to revel in parenthood quite the way he did. He was destined to be a dad and boy did he excel in every way. Just a few weeks ago, we were on the phone excitedly discussing Gus's arrival and he mentioned that he wanted to open a swim school for kids and he was excited to teach him. Sigh. Life is wild."
"Keeping his partner, son and the legion of pals he had in my thoughts and prayers. Today, we're going to crank some Grateful Dead and celebrate you, friend. Love and miss you, always GG."
Many of Mandy's friends sent their condolences on Instagram, including Chrissy Metz and Rachel Bilson.