Whenever it’s possible.
By the way, although lenition is often marked with h after a consonant, it is a phonological process that occurs to some consonants regardless whether it’s marked or not. One of them is slender l which is lenited in speech, although not marked in writing (one would write glé leathan but pronounce the l in leathan differently, see this article on Akerbeltz wiki). Traditionally glé luath would also have different l than luath on its own but the distinction disappeared in modern Gaelic dialects according to Akerbeltz.
In general lenition is a process of making consonants weaker, that is less consonant-like and more vowel-like: stops change to fricatives, some fricatives (s) become h, others (f) disappear entirely. Originally it was a purely phonological phenomenon (depending only on the phonological context) that happened to single consonants standing between two vowels (that’s why some combinations of consonants, like sg- never get lenited), but later the language lost some vowels and it became conditioned by grammar rather by phonology later in Celtic languages.
Well try doing the Irish and Welsh courses.
All Celtic languages have slightly different rules, and Gaelic and Irish have the special orthography with the h not even shared with Manx.
But the real killer is that it's not just one mutation. One in Scotland but as you go south it gets worse. Two in Ireland and the Isle of Man. Four in Wales. About five (?) in Cornish and Breton.
You have actually drawn attention to some important points here.
Firstly, only the first part of the course has been written so far, and this is not mentioned in the notes, as it is not usually mentioned to beginners. The only tiny clue that all might not be what it seems is where it says
Singular feminine nouns usually cause this lenition (in writing) in adjectives starting with the consonants [...]
but that is pretty subtle. Next, to be really pedantic, and to contradict the notes, lenition is actually the sound change, not the orthographic change. So adding the h is not lenition. It is the way lenition is marked in writing.
The next problem is that people are more and more learning language in writing. Even in English, I increasingly hear words pronounced the way they are written, instead of the correct way. People are losing the knowledge of how to pronounce things that do not match the spelling, so unwritten lenition may well die out.
The actual sound change for l is beyond this level of the course and I do not want to rush ahead, but silmeth has referenced an article if you are interested.
In virtually all cases in Gaelic, though not in other Celtic languages, the insertion of the h, representing lenition, is caused by the previous word. Apart from feminine nouns (where the lenition is still technically caused by the previous word) always look at the previous word.
Glè is simply on the list of words that cause lenition. Eventually you will get the habit of adding words to your mental list when you see lenition occurring. Not sure what you have met so far. Maybe dà 'two'. Maybe one or two adjectives that go before the noun. The details are not important. The important thing is to learn when you see or hear one.
The annoying thing is that dictionaries, including the Duolingo one and several good dictionaries do not mention this. It should be the first thing you see when you look up any word that causes lenition. There is no obvious solution to this. But at least you could make a list when you see them.