The glyph is not a composition. Its width in East Asian texts is determined by its context. It can be displayed wide or narrow. In bidirectional text it is written from left to right. When changing direction it is not mirrored. The word that U+03B3 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with one other glyph.
The Wikipedia has the following information about this codepoint:
Gamma (; uppercase Γ, lowercase γ; Greek: γάμμαgámma) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop IPA:[ɡ]. In Modern Greek, this letter normally represents a voiced velar fricative IPA:[ɣ], except before either of the two front vowels (/e/, /i/), where it represents a voiced palatal fricative IPA:[ʝ]; while /g/ in foreign words is instead commonly transcribed as γκ).
In the International Phonetic Alphabet and other modern Latin-alphabet based phonetic notations, it represents the voiced velar fricative.