Loring tells PEOPLE the family will get together on Saturday: "It will very bittersweet, but I think being together is very healing"

Alan Thicke‘s death has left his Hollywood costars and fans reeling — but it struck his family the hardest.

According to the late actor’s first wife, Gloria Loring, their sons Brennan Thicke, 41, and Robin Thicke, 39, were left speechless after their father died of a heart attack at age 69 on Tuesday.

“My son Brennan sent me a text that said: ‘Dad’s had a heart attack — it’s a big deal,’ ” Loring, 70, tells PEOPLE. “I called back but he didn’t answer the phone. Then I got a call from Robin after that. Robin said that they had taken him to the hospital and that he was going into surgery. I said: ‘Well, that sounds hopeful.’ Literally minutes later, I got a text message from Brennan that said: ‘He’s gone.’ ”

“When I read it, I said out loud: ‘No.’ At first, I couldn’t believe it,” continues Loring. “I thought: ‘What? No. It can’t be true. It’s too fast. It’s too final.’ ”

Loring says as the day went on, her two sons were “just devastated and could barely talk.”

“I’ve sent them some messages and have called — neither of them are answering their phones right now,” she adds.

Credit: Ron Galella/WireImage

And while the family grieves the loss of the Growing Pains star, they will get together on Saturday — a plan originally set in motion to celebrate Loring and Brennan’s birthdays. (Loring’s was last Saturday, and Brennan will turn 42 this Saturday.)

“We have family members who were flying in for that — so we will be able to have a little commiserating celebration of Alan,” she says. “It’ll be a remembrance. Obviously, we have to continue to embrace what we do have and there’ll be a lot of talk about Dad. It will very bittersweet, but I think being together is very healing.”

As for how her two sons are coping, Loring says that would be “their story to tell” — and while Brennan has not yet publicly commented on his late father’s passing, Robin took to Instagram to share a heartfelt message shortly after news broke on Tuesday.

“My father passed away today,” he wrote. “He was the best man I ever knew. The best friend I ever had. Let’s all rejoice and celebrate the joy he brought to every room he was in. We love you Alan Thicke. Thank you for your love. Love, your grateful son.”

Loring and Alan were married from 1970-84 (they separated in ’84 and their divorce was finalized in ’86). The actor also has a third son, 19-year-old Carter Thicke, with his second wife, Gina Tolleson. The two were married from 1994 to 1999, and Alan later married his third wife, Tanya Callau, in 2005.

Loring says she feels particularly emotional for Callau, as well as Carter, with whom Alan was playing ice hockey when he suffered the heart attack.

“We all feel so devastated for Tanya and for Carter,” she says. “They’re living with him every day. That physical loss — we have the emotional and psychological loss — but for them to also have the physical loss of his presence and his humor and his perspective and wisdom and guidance … my heart goes out to both of them so much.”

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Ultimately, Loring says that while Alan was obviously “a very hard worker” and “very devoted to his creative life,” he will always be remembered, above all else, as a loving and devoted father.

“He once planned a trip and took the boys up to be with Wayne Gretzky’s family,” she recalls. “They went up to Canada. Being a Canadian was at the center of his being. He took them up there with Gretzky’s family and they went camping and canoeing and I just remember that — that was just the kind of dad he was. He wanted his kids to have those great experiences.”