Flygp meets the regulatory requirement to prove your performance to 1500 ft, nothing more, unless you specifically request it. If you are below the max mass quoted you may use tab data for that weight, as the speeds given by Flyg are for the max performance (or structural, if lower) mass quoted.
It is up to you to calculate if you will still be able to comply with the SID. On that note, it is well worth looking at why the gradient is required. Many SIDs have a gradient for ATC purposes, some (swiss places in general) are for terrain clearance. Logically, if I am OEI on a 2 engine aircraft, unless weather, performance or special circumstances preclude it, I am probably going to return for an overweight landing where I just left. Having of course taken time to suitably prepare, which I will do at an appropriate safe altitude/position.
Where all this gets a little muddy is what you do if you have already turned into the SID when life starts to get interesting, but that's why we get all that extra commanders salary isn't it??!
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