SIDs climb gradient requirements (standard or higher) are based on all-engines operating. There is no requirement to let ATC know beforehand if you can't meet the gradient of a published SID in the event of an engine failure. Some SIDs have crossing restrictions well into the Flight Levels and almost no engine-out civil aircraft could ever meet them. Southeast-bound SID out of Geneve with winter tailwinds, for example.
The regulatory gradient requirement you must meet for every takeoff whether VFR or IFR regardless of terrain or the existence of published SIDs (assuming you're flying a 2-engine aircraft) are those required for certification, and usually the most restrictive in terms of adjusting weight to meet it is the 2.4% 2nd-segment climb gradient after the loss an engine (not taking into consideration approach and landing climb gradient requirements at your destination, which may be more limiting as far as your takeoff weight is concerned).
That being said, if you're departing in IMC conditions and you know you can't make the published SID climb requirements to the airway MEA or at least the MSA, what do you do if you lose an engine?
If you're a commercial operator you'll have an approved engine-out procedure for that aircraft type that may involve turns (taking into consideration 15 degrees of bank) and an acceleration altitude (where the 2nd segment ends). They're escape procedures designed around known obstacles and terrain. The result of having these procedures available is being legal for far greater takeoff weights at many airports.
If you're a non-commercial operator and it's IMC (or virtual IMC in CAVOK darkness) and you can't see to avoid terrain or obstacles, if you can't make a published procedure to at least the MSA by the end of 2nd segment as determined by engine time limits or lack of performance numbers to altitudes that high above the departure runway, a big flag should go up if you don't subscribe to a service that can provide an engine-out turn procedure for that runway that's been approved for your aircraft. Don't blindly rely on ATC to keep you out of the dirt, and never assume you can fly-straight out forever clawing for altitute even if you think terrain isn't an issue.
Also remember, very few aircraft manufacturers provide single-engine climb performance numbers based on anything other than a max 15 degree banked turn. Where terrain is an issue, to achieve a contained-area turn many SIDs require not only a max speed but also require minimum bank (say, 25 degrees) to acheive desired, small-radius ground track. In those cases, you really don't have any gradient numbers that take into account the degradation in climb performance for an engine-out, V2 climb, 25 degree-banked condition.