You’re mixing together two different concepts: endogenous GH secretion and injected GH.

High glucose/insulin can suppress your body’s own GH pulse, which is where a lot of the “take GH fasted and don’t eat carbs around it” advice comes from. But once you inject GH, insulin isn’t going to somehow cancel that GH injection. So I wouldn’t be losing sleep over whether eating carbs shortly before a bedtime GH injection is “wasting” the GH.

Where I’d look at it differently is glucose control. GH acutely reduces insulin sensitivity and glucose uptake, and chronically higher GH exposure can worsen glucose tolerance. So pounding a huge carb meal immediately around GH may not be ideal metabolically, even if it doesn’t meaningfully negate the GH itself. The reta complicates the timing even more. Retatrutide can delay gastric emptying, so “I ate the carbs two hours before GH” doesn’t necessarily mean those nutrients are finished entering circulation when you inject. The degree of gastric-emptying delay also isn’t necessarily constant over long-term. As for the metformin: I don’t see online any evidence that taking metformin at the same time as GH meaningfully “blunts” injected GH. There are actually human data where GH and metformin were administered together and IGF-1 still increased substantially. Across randomized trials, metformin hasn’t shown a consistent clinically important reduction in IGF-1 in adults either. So I wouldn’t rearrange my entire eating schedule just to manufacture a long “fasted GH window.” If getting your calories/macros in requires eating closer to bedtime, I wouldn’t assume you’re throwing away the GH. Personally I’d be much more interested in overall glucose tolerance over time than whether the last carb was 60, 120 or 180 minutes before the injection.