I ni ce!
ߌ ߣߌ߫ ߗߋ ߸ ߖߋߙߊߡߌ߫
Cool to hear that you are diving into N’ko 
Your question about the g sound is a good one. And the answer is slightly complicated.
Short Answer
First, you are right: you don’t want to use ߜ {gb}.
Instead; for now, you should use the equivalent of k (ߞ):
ߒ ߕߐߞߐ ߞߏ ߖߋߙߊߡߌ
This isn’t “correct” N’ko spelling technically, but it’s a good place to start and even native speakers start out doing this like this when they are mostly “transcribing”. Later, you will learn how to “spell” properly.
(NOTE: That when representing foreign language words that require “g” specifically, N’ko calls for using ߜ with a dot diacritic on the top: ߜ߭, but I wouldn’t recommend that for writing a Manding word for reasons laid out below.)
Long Explanation
In short, g is not part of the “sound inventory” of Manding in the N’ko tradition. For you as someone who started with Bambara in Latin script, this may seem a little weird, but let me explain.
In general in Manding, “intervocalic velars” (that is, sounds like k, g, x, etc., that appear between two vowels) are non-contrastive. What I mean is that changing the sound between two vowels in a word like tɔgɔ (using Latin-based Bambara orthography) does not alter the meaning. For instance:
tɔgɔ
tɔkɔ
tɔxɔ
tɔɔ (that is, with no “velar” sound between the ɔ’s)
‘name’
All the different pronunciations mean the same thing.
This a little bit like in English with a sound like R. We can use an American R or a rolled Scottish R in a word like “round” and it makes no difference, despite the fact that the sounds are different. They are “non-contrastive”.
In the N’ko tradition, the approach to this reality is actually to write such words with no intervocalic consonant at all (but with a diacritic above the vowels to represent lengthening [as well as tone, but let’s leave that aside]). For instance:
ߕߐ߮
In strict transliteration this could be:
{tɔ́ɔ̂}
(You may want to ignore the tonal diacritics that appear above the vowels for now. Just note how the single vowel ߐ becomes two ɔ’s in transliteration)
In N’ko teachings, this word can be pronounced orally as “tɔɔ”, “tɔgɔ”, “tɔkɔ”, “tɔkɔ”, etc.
If in N’ko though, you want or need to mark the fact that there is definitely a velar sound between vowels (like in the word fanga ‘strength’ [using Latin-based Bambara spelling], then the typical approach would be to actually use the equivalent of k (that is, ߞ in N’ko). For instance:
ߝߊ߲߬ߞߊ
In strict transliteration, this would be:
fànká
But of course, in the Latin-based Bambara tradition, they opt to write this as:
fanga
So basically, the N’ko-based and the Latin-based traditions have two different approaches and/or conventions that are related to their underlying conceptualization of the language.
If you want a fuller explanation, perhaps consider checking out the section labelled “Logographic Unity” on p. 209 of my dissertation (which addresses the ߜ issue) or my article “Orthography, Standardization and Register: The Case of Manding”, which lays out the philosophical differences between N’ko and Latin orthography for Manding.
Hope that’s helpful!