In traditional British academic English, this name for the rivers of ice found in frozen mountains, the Arctic and the Antarctic and so on, was pronounced with an '-s-' sound in the middle, and with the first vowel like that in 'cat' and 'that': 'GLA-ssy-er', IPA: /'glæ sɪ ə/. American English makes the first syllable rhyme with the first syllable of 'facial', and makes the central consonant sound a '-sh-': 'GLAY-sher', IPA: /'gleɪ ʃɪ ə/.
Good speakers will adapt themselves to the context in which they find themselves, so you would be well advised to use the normal pronunciation in your department. If this is not clear, or is varied, you must choose. AWE would encourage you to use the traditional British pronunciation in such a case.