does this mean both "he is going somewhere" (you don't know his destination) and "he is going (walking) somewhere" (you don't know where he is walking at the moment)?
Generally: to walk = chodzić (often spacerować, meaning a stroll); to go (somewhere) = iść (gdzieś). So 'He is walking somewhere' would be 'On gdzieś chodzi' (kind of weird sentence, maybe when you want to say he is walking around without any sense or aim) or 'On gdzieś spaceruje' (he's taking a stroll).
Also note that 'gdzie' in those sentences with 'idzie' and 'chodzi/spaceruje' has different meanings: iść gdzieś/dokądś (like German wo/wohin, in English it's similar)
In English this has two meanings: 1. Travel and movement 2. Aspirational
Does the second meaning of to go (aspirational) exist in Polish? "He's going somewhere," "Everyone always knew Sara was going places" Referring to someone pursuing a goal, career, etc.
In such figurative sense you should rather use "zmierzać". "On gdzieś zmierza" (He is going somewhere).
i would imagine such a complex language as polish would have alternative words for going somewhere and being somewhere but it is "gdzies jestem" and "gdzies ide" ?!
Usually, the newest, most important information is at the end of the sentence (or is somehow stressed by voice modulation), so your sentence focuses on the 'somewhere' part - he is going, but it's not important where. But the versions are not that different.