The way I pronounce it, they sound the same, but I wanted to know from native speakers. Don't wanna run the risk of saying the later when I mean the first if it is actually different.
They're similar enough that I can imagine someone mistaking one for the other if they were listening to a low quality recording, somebody with a strong unfamiliar accent, somebody who is drunk and mumbling, etc. But I wouldn't say they are exactly homophones.
It's obviously difficult to make sweeping statements about every English accent ever, but I think the vowels in "just" and "dis" would rarely be the same, and to English native speakers I think the "j" and "d" are unlikely to be confused either.
Although many colloquial Englishes drop "t"s, and the "t" at the end of "just" is likely to disappear completely, in the word disgusting I can't imagine the t vanishing altogether. It would more likely be softened into a "d", but I would think is still clearly distinct from the complete absence of t/d before the "ing" of guessing.
While the vowel sounds of guess and gust are clearly distinct to me, I can imagine some accents where they are more similar.
- /aɪm d͡ʒʌsˈɡesɪŋ/ (with a dropped "t" at the end of "just" blended into "guessing")
- /aɪm dɪsˈɡʌstɪŋ/
As you see, although the sonsonants are similar, the phrases are not homophones, and even in quite quick speech, I'd expect few native speakers would mishear these phrases.
In particular the stressed vowel in guessing and -gusting is quite different. The consonants /d͡ʒ/ and /d/ are also phonemically distinct, as are /s/ and /st/
It is very unlikely that these would be misheard, as they aren't likely to appear in the same context.