Gameboy can't change square wave volume without resetting the phase of the wave (clicks and metallic buzzing). It can do it in a limited way with volume envelopes though, it can strictly decrease or increase volume at a regular rate with no buzz artifact.
It's a different beast entirely.
So your GB volume envelopes would need to be either increasing/decreasing by 1, and it could take up to 7 frames to go up or down by 1, but must be a regular interval. So you could do an envelope like 10 11 12 13 14 15 15 15, or 10 10 11 11 12 12 13 13, or 8 8 8 7 7 7 6 6 6 5 5 5, but if the envelope does not fit that form, you do a phase restart (pop/buzz) to proceed to the next part of the envelope.
Also it's not truly frames as the time unit, but 1/64th seconds, similar enough.