According to Muslim belief, based on Quran, Gabriel is the name for "Holy Spirit" the dearest Anglel of God. and it is not the Prophet. it is also mentioned in Quran as (Angel) "rouh" , meaning spirit.
About John 14:16, the verse never mentiones that the comforter is Holy Spirit. if it is a Christian interpretation of the verse (John 14:16) I dont know about that, but Muslims say that this comforter is Mohamed(pbuh).
If Christians believe that "comforter" is Holy Spirit, and Muslims believe that "comforter" is Mohamed(pbuh), this does NOT rmean that Holy Spirit is Mohamed(pbuh)! You are reffering to oposite sides to get a result.
Regarding 2:97 and 16:101, and "confirmation" of the earlier revelations and at the same time "substituting" them, Quran does confirm earlier revelations (not the changed version of them of course!), and please note that the word translated as "substituting" means rather "transforming" and not substituting . which means updating", point is, Islam is the religion of Adam (pbuh) to Mohamed (pbuh), but in different levels, it was "updated" untill it reached the final form which was brought by Mohamed (pbuh). so while maintainig the same spirit, it was "transformed" to what it is now. I hope this explanation makes the situation clear!
About Quran being in "pure" Arabic. yet another misunderestanding of Quran caused by looking a "translated" copy rather than Arabic! the word "mobeen" means clear with explanation, understandable, NOT pure (in the way you percieve). and of course it is not in pure Arabic. There is no "pure" language at all! in every language you can find the trace of other languages, older or contemporary ones.