Hi Colin - it depends on which pronunciation you sre going for. The reconstructed Roman pronunciation which is now taught to students would have it with both Gs as in go, and the ‘i’ in sidus as you suggest.
However, wikipedia tells me that Georgium Sidus was the name originally given by William Herschel to Uranus at the end of the 18th century, so the traditional English pronunciation would seem to be better - with ‘G’ as in George (extra appropriate, as it was named in honour of George III). I think the “i” in Sidus would have been pronounced like the English letter (i.e. like “eye”), but I could be wrong about that.