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Re and Im work properly, with appropriate assumptions, in the example like this

Assuming[g[_, _] ∈ Reals, Simplify[Im[3*I*g[r, r2] + 45]]]

On the other hand, if the derivative of the function is also present, similar approach does not work

Assuming[(r | g[_, _] | D[g[_, _], _]) ∈ Reals, Simplify[Im[3*I*D[g[r, r2], r] + 45]]]

i.e. does not give back 3*D[g[r, r2], r]

More dramatically,

Assuming[(r | g[_, _] | f[_]) ∈ Reals, Simplify[Im[3*I*(D[g[r, r2]*f[r], r]) + 45]]]

gives 3 Re[g[r,r2] (f^′)[r]+f[r] (g^(1,0))[r,r2]].

In real problem, I have the function of four variables and mixed partial derivatives, so it would be great if there is some generic way to prescribe all of function's derivatives as Real.

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Use the FullForm of the derivatives:

Assuming[(r | g[_, _] | Derivative[1, 0][g][_, _]) ∈ Reals, 
   Simplify[Im[3*I*D[g[r, r2], r] + 45]]]

enter image description here

Assuming[(r | g[_, _] | f[_] | Derivative[1, 0][g][_, _] | Derivative[1][f][_]) ∈ Reals, 
   Simplify[Im[3*I*(D[g[r, r2]*f[r], r]) + 45]]]

enter image description here

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    $\begingroup$ Great, it works! Form Derivative[_, _][g][_, _] allows for generalizing to arbitrary number of derivatives. $\endgroup$
    – Thela Hun Ginjeet
    Commented Sep 23, 2018 at 10:50

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