Story highlights First lady Michelle Obama tweets: "I am so happy for you and Amber!"

Roberts has not previously spoken publicly about her sexuality

The ABC anchor thanks "my long time girlfriend, Amber" in a Facebook message Sunday

"At this moment I am at peace and filled with joy and gratitude," she writes

"Good Morning America" anchor Robin Roberts used a Facebook post to acknowledge what friends and co-workers have long known: She's gay.

Roberts, 53, has been very open about her health battles, but she has not previously spoken publicly about her sexuality.

First lady Michelle Obama tweeted her support to Roberts Monday: "I am so happy for you and Amber! You continue to make us all proud. -mo"

In an end-of-the-year message posted on her Facebook "like" page Sunday, Roberts wrote: "I am grateful for my entire family, my long time girlfriend, Amber, and friends as we prepare to celebrate a glorious new year together."

While Roberts did not give her girlfriend's last name, People magazine identified her as Amber Laign, a licensed massage therapist from San Francisco whom she met through mutual friends a decade ago.

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It was ABC's Roberts whom President Barack Obama chose in May 2012 for his landmark interview to acknowledge his support for same-sex marriage.

Roberts has kept few secrets about her health. She wrote and spoke extensively about her battle with breast cancer in 2008 and again in 2012 when she underwent a bone marrow transplant to treat a rare blood disorder called myelodysplastic syndrome.

Sunday's Facebook posting came on the first anniversary of the 100th day of her recovery from the transplant.

"At this moment I am at peace and filled with joy and gratitude," she wrote. "I am grateful to God, my doctors and nurses for my restored good health. I am grateful for my sister, Sally-Ann, for being my donor and giving me the gift of life. I am grateful for my entire family, my long time girlfriend, Amber, and friends as we prepare to celebrate a glorious new year together. I am grateful for the many prayers and well wishes for my recovery. I return every one of them to you 100 fold. On this last Sunday of 2013 I encourage you to reflect on what you are grateful for too."