The Sounds of the Letter g
The letter "g" comes close to following a phonics rule similar to the one for the letter "c" in that it is always pronounced /g/ unless it is followed by an "e," "i" or "y." So, while it's true that the letter "g" makes the /j/ sound only when followed by "e," "i" or "y," you can’t be certain that it will be pronounced /j/. In some such cases "g" still makes the /g/ sound.
Words with "ge" usually follow the rule, but then we have the very common word get, plus words like rugged and shrugged. The "gy" combination usually works, but it isn’t a very common occurrence and then the "gi" words are all over the lot, with giant, ginger, and gist offset by words like gift, gill, and give.
Because of the much lower reliability of the pattern in the case of the letter "g," it is not advisable to teach a hard and fast Rule of g. However, here is a worksheet that makes the point that the letter "g" represents two different sounds, /g/ and /j/.
Note that the above worksheet does not refer to a phonics rule governing which pronunciation to use in a particular situation. You can decide whether or not your child is old enough to understand the somewhat different circumstances about when "g" is /g/ and when it is /j/. You should, in any case, make sure that your child understands that he should be prepared to try both sounds in an unfamiliar word.
Incidentally, one additional reason not to teach a phonics rule for the letter "g" is that the initial blends "gl" and "gr" are difficult to pronounce using the /j/ sound for the letter "g" (jlad? and jrip?) unlike the case with the letter "c" where "cl" could be pronounced “sl” and a word like clip could be misread as slip.
If you find this lesson useful, consider returning and investigating the OnTrack Reading Advanced Code Phonics Workbook. The workbook is a complete advanced code phonics program that incorporates each of these "Tidbits" and covers precisely the information your child will need to become proficient in phonics, including the highly effective OnTrack Reading multisyllable decoding method.