Wondering when and why historically the Anglo-Saxon letter "Y" became a (part-time) vowel substitute for the letter "I", leading to "gymnasium" instead of "gimnasium" or "cyanide" instead of "cianide" etc.
The answer is that Y has always been used as a vowel in English.
For example, hyð. The OED has a citation from Corpus Glossary (c736):
Deconfugione, statione, hyðae.
And another from Metres of Boethius (1000):
Þæt is sio an hyð.