"That is guaranteed."
Translation:Das wird gewährleistet.
You can say and you will hear "das ist gewährleistet". About what you call "subtle difference", for me this seems like a hard question. The verbs meaning makes it complicated. The "action" is somehow not entirely in the present but reaches into the future, and I guess that this makes it difficult in certain situations.
Think of it as, "That is being guaranteed by someone." This is the passive voice and guaranteed is being used as a verb (past participle). In "That is guaranteed," guaranteed is being used as an adjective (but still as past participle). I am using "being" to emphasize the difference between the two phrases in English. I would not normally add it to that phrase: "That is guaranteed by someone." German solves the problem by using wird and ist.
I think the literal translation is: Das wird gewährleistet werden.
But am not a native speaker, and am just following this learning material: http://www.dartmouth.edu/~deutsch/Grammatik/Passive/Passiv.html
In practice the present tense seems to be used a lot for the future, so it could just be "Das wird gewährleistet" (same as "that is guaranteed"). Examples: https://context.reverso.net/translation/english-german/will+be+guaranteed