/i:/ is the vowel that we find in the word FLEECE. I put that word in capitals because that is how that vowel is often referred to by linguists: the fleece vowel - or FLEECE for short. (This is not random, the word was specifically chosen for a number of specific reasons.) It is the vowel sound at the end of the word guarantee. In transcriptions of British English it has a colon [ : ] in the symbol to describe the length.
/ɪ/ is the vowel in the word KIT. It is known as the kit vowel - or KIT for short. It is the vowel we find in prefixes and suffixes, the bits we stick onto the beginnings and ends of words. So, for example it is the vowel we hear in --ing verb endings.
The vowel represented by /i/ at the ends of words in dictionaries is usually referred to as the happy vowel - HAPPY. This vowel may sound like either FLEECE or KIT, but is always short in duration.
If you say the < y > sound that we find in the word yes, and then say the < e> we find in the word end, the kit vowel is somewhere between the two sounds. This is the first vowel in the word infinitely. This word would sound very odd to a native speaker if it was said with a fleece vowel, /i:/! It would sound like a made-up word: eenfinitely.
The Original Poster asks if there is a big difference between these vowels. If we are talking about the physical difference between the sounds, the answer is: no. In fact, it is very unusual to have two vowels that are so similar in one language. They are very close together. In most languages these would count as one vowel. However, if we are talking about the meaning, or the effect on a listener, the answer is: yes! There is a big difference. There are very, very, very many words that we can be confused about if you say the wrong vowel. For example, the words peace and piss. Nobody wants to say Piss man!, when they mean Peace man!.
If you want to type IPA script, this website is very useful
Hope this helps,
Peace!
Best Answer
The pronunciations of proper names are notoriously unpredictable.
So where do you turn to learn the “correct” pronunciation?
You could follow the usage of knowledgeable users ... But in your case, unfortunately, these users are not consistent.
You could assume that the spelling follows ordinary patterns. This is, to be sure, a very unreliable guide. But a little poking around in rhyming dictionaries turns up the fact that English words in -adle—cradle, ladle—are pronounced with a ‘long a’, /eɪ/, while words with a ‘short a’, /æ/, are spelled with a doubled consonant: addle, saddle.
Your best source is “the horse’s mouth” (if you don’t know this expression, which means ‘the authoritative source’, it is explained here). In the end, the pronunciation of a name is determined by the person or entity who bears it. I find nothing explicitly about pronunciation on Gradle's own website, http://www.gradle.org; but I do find mention of a presentation “Rocking the Gradle”, which is very suggestive. We speak of ‘rocking’ a baby’s cradle, and the pun would be much less effective if Gradle and cradle didn’t rhyme.
I also find that the originator of the system and the organization is named Hans Dockter, who we may presume is the “horse”. Searching on that name on YouTube I find a video interview with Herr Dockter here—and in it he consistently pronounces the name to rhyme with cradle and ladle.
Just as a by-the-way: the word gradle (no capital) is in fact recorded in English, though I have found it in no dictionaries. It appears three times in Sean O’Casey’s play Juno and the Paycock, where it represents a Dublin working-class elision of the quantifier great deal—“I’m sorry to say there’s a gradle wrong with her.”