Gi gu is the first of the four Tibetan vowel signs, the dbyangs, and writes the vowel i. It is placed above the consonant it modifies and replaces that consonant inherent a, so ka with gi gu upon it is read ki.
It is a dependent sign and not a letter. It cannot stand alone, it is not one of the thirty, and where a syllable begins with the vowel i the sign is written on a chen, the thirtieth consonant, which exists in part to give such vowels something to attach to. Shown on its own, as in a code chart or on this page, the sign is displayed on a dotted circle, which is the convention for a mark that has no independent position.
The four signs are recited after the thirty consonants, and the thirty and the four together are what a Tibetan child learns as the alphabet. Four is a small inventory, and it is small because Tibetan has a small vowel system and does not distinguish length. Everything a longer inventory would be needed for, in particular the long vowels and diphthongs of Sanskrit, is handled by the transcription apparatus that sits outside the alphabet.
The four signs are recited in a fixed order, i, u, e, o, and gi gu comes first. That order does more than govern the recitation. It is also the order in which the vowels are compared when Tibetan words are sorted, once the root letter, the superscript and the subscript have been taken into account, so the sequence is part of how a dictionary is arranged.