Ardex on Gypcrete Subfloor

I'm prepping a 3/4" light-weight cement subfloor (could be gypcrete, but not sure) in a 2nd story condo for floating an engineered wood floor. Subfloor is in good shape with only small cracks, but it's not smooth enough, with deviations in some areas up to 1/2" max. (The woodfloor specs require maxiumum 1/8" deviation over 10 ft). I'm considering using Ardex GS-4 self-leveling compound, but the minimum application thickness is 1/4". Ardex has another product for thinner applications and feather-finishing called SD-F, but it's a portland cement, and I'm not sure it's suitable for a gypsum subfloor (assuming the subfloor's gypcrete). Does anyone have experience with either or both of these products and suggestions as to which would be more appropriate? Will the GS-4 stick in areas where it's not 1/4" thick?
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