Segments are not intrinsically hard, or easy, to produce, so there is no ranking of segments. It may be possible to derive a subject pool-specific ranking, where a pool of subjects have a difficulties with {a,b,c} but not with {d,e,f}, and the content of those sets is related to the languages that they speak. Difficulties may be perceptual or they may be articulatory, or both, and the two causes are often related.
Certain people have difficulty with [θ,ð], though generally not English, Greek or Icelandic speakers. You can detect this by observing non-fluent productions of English where "this" may be produced as [zis] or [dis]. There is also a common problem with [r] and [l], a speaker of another language may mistake "lip" and "rip". Perception difficulties are more common, and are often the cause of articulatory errors. Because of this chicken-and-egg question, it is generally pointless to focus just on articulatory mechanisms.
There has long been a belief (not totally crazy) that there is an ordering to segments, whereby that t is more basic, indispensible, easier or however you want to put it, that d and d is better than t'. This notion is embodied in the theory of what is known as "markedness". There is no clearly agreed-on ranking of segments (consonants and vowels are generally treated as completely unrelated), but one might for example be able to order the IPA to reflect a hierarchy like t>k>p>q>ʈ>ʢ, generally based on frequency of occurrence across languages. The general (and extremely wide-ranging) topic of segmental markedness will most likely yield a context-free ordering of segments, but applicability of such a ranking will face language-specific quirks such as the lack of /p/ in Arabic and the presence of rather "marked" /tˁ ʕ ħ/.
The example that you provided of /b/ /g/ /t/ being harder that /b/ /a/ /t/ indicates yet another problem, that you are interested in certain sequences of segments and not just the segments /b/ /g/ /t/ is a list of three sounds (the order doesn't matter), and /b/ /a/ /t/ is another list of three sounds – mostly overlapping the first set. Therefore what you must mean is that you think the sequence /bat/ is easier than the sequence /bgt/. This, fortunately, is also reasonably well studies, though is generally about perception and not about production. Phonologists often organize sequences of segments into syllables, which follow rules, and as a general rule, a syllable is centered around a vowel, with some preceding and perhaps following consonant(s). /bat/ can be syllabified as is in English, but not in Logooli (nothing after the vowel in the syllable). English allows /brat/ but many languages (which have all of the segments) don't allow initial clusters (Logoori is an example). Reading up on syllable structure and the notion of "sonority sequencing" will reveal some of the main generalizations (and the controversies).
Articulation can sometimes be brought to bear and given a bit more advantage in constructing an explanatory account for a fact. There is no difficulty in producing the set of constrictions in /bgt/, but there is an aerodynamic-physiological challenge in making /g/ overlap /b/ so that there is no release of air in the transition from /b/ to /g/. This is compounded by the need to keep pumping air out of the lungs to produce the span /bg/. We don't do this in English so we don't have articulatory practice with such a sequence, thus it is most likely that this would be produced as [bɨ̆gt].
If you aren't considering how segments are modified according to context, then it's very non-obvious that you can model anything validly. Absolute markedness is pretty unimportant in language change, it's all about context.
Xhosa doesn't have glottal stops, so that will simplify things, unless you mean something besides glottal stop.
