'K' and 'ch' never make a 'g' sound, at least that I can think of.
In those examples, they both make a 'k' sound. However, try saying 'sk' and 'sg' back to back. You'll find the sounds will be indistinguishable if you say the 'sk' quickly. It's still definitely a 'k' sound, but if you say a 'g' no one will notice if you speak at a normal speed.