What a great story. I like her.
My Grandma was a smokin' and drinkin' flapper girl in the 20s. At 18 she married a man more than twice her age just for his money but learned to love him because he actually was a great guy. Spent every day at the yacht or golf club then went for martinis and dinner parties every night. She loved her kids but had no desire to be a mother so hired an awesome life-long mother for her two boys.
She was so much fun. Far before anyone normally learns such things, she taught me how to shoot pool, golf, drive a boat and mix a martini.
I don't think she ever told me she loved me or me that I loved her but we both knew. I was always honest with her and she did not care if I did a lot of drugs or was a complete slut but she was brutally honest in a good way and completely supportive.
I cried like a baby when she died at 91. True to her character, she had a boyfriend she was banging who had the genuine old man name of Horace Greeley.
It sounds like you have had a great relationship. Hail to your Gram. If you are so inclined, there is no downside now in getting "cutesy-gooey". Perhaps just be honest and retell some of the things you have said here. How you respect her for what she has done. How she has been a role model. etc. Even if she pretends she does not like it I bet she will. And there is not a chance in hell that you will regret it.