A J-sound can be written in different ways in Swedish. It’s pretty easy to understand though, when you see that the words come from other Germanic languages, some still using the “old” pronunciation.
By göra we mean the preterite form gjort.
There is no other explanation that these sounds are pronounced with a J-sound than time and nature have developed the language. The same thing is true for göra, to do, where G is pronounced like a Swedish J. More about this in lesson 8. D, G, H and L are all mute and you will just have to learn the pronunciation. At sayitinswedish.com you will see this in the phonetic text which will begin with [j] to mark this behaviour.