Codilty, binary gap get the highest gap
It would have been a good idea to link to the problem statement in your question.
Please run gofmt on your code, typically have your editor automatically format your files every time you save your source file.
Your naming conventions aren't very Go-like.


Solutions is too vague, BinaryGap would be better.
convertNumberToBinary is really long; this isn't Java — toBinary would largely be enough
stringArray is very wrong: this variable is a byte slice, plus, it doesn't give you extra information from its type
TestBinryGap should probably be TestBinaryGap
testObjects is weird and slightly misleading; Go test files usually name this variable test or tc, and expected is pretty much always called want

When you want to append repetitively to a string like in convertNumberToBinary, use string.Buffer. But here, why not just use a []bool that you can name isOne or something?
I think the function is simple enough that you can just put it in one function, and compute the binary representation at the same time you compute the binary gap. This would make your memory complexity $(1) instead of O(log(n)) (no need to allocate a slice or a string anymore) and simplify your code.
You don't need to convert your string (or your []bool) into a []byte later on; you can just iterate on it directly.
You should probably test that the input is a nonnegative number and return an error if that's the case (although maybe this isn't necessary in a competitive programming setting).
On the positive side, your test is a good idiomatic example of struct-based tests =)