What is the cantonese phrase for "forever grace"? I know it's not 永远的恩典 (Yǒngyuǎn de ēndiǎn) so don't try google translate.?

Yes, most Chinese people, which might include me, cos I'm Taiwanese, are not good at English. But Google Translate is more worse than we could do.
Let me make an example first, if someone get an award, like Nobel Prize, he is deserved to be complimented forever. And yes its's indeed, I think the word "grace" used in this phrase should be changed to "honor/榮耀" and "forever" should be changed to "eternal".
恩典 doesn’t have the same meaning as 榮耀.
For example, someone was a poor orphan boy in war districts and he was taken to the US by his American parent-in-law and has grown up to be an important person. When he makes a speech he might say my parent-in-law is my eternal grace (永远的恩典/Cantonese phrase: forever grace). - This time Google Translate almost did a wondeful job.

"Forever grace" doesn't make any sense. "Forever" is an adverb, but "grace" is a noun, and an adverb can't modify a noun.

Do you mean "eternal grace," maybe? "Endless grace"?