For gdk_rbga_parse, we have:

/**
 * gdk_rgba_parse:
 * @rgba: the #GdkRGBA to fill in
 * @spec: the string specifying the color
 *
 * Parses a textual representation of a color, filling in
 * the @red, @green, @blue and @alpha fields of the @rgba #GdkRGBA.
 *
 * ...
 */
gboolean
gdk_rgba_parse (GdkRGBA     *rgba,
                const gchar *spec)

So it looks like the argument rgba should be annotated with (out).

I hesitate to say (out) is definitely missing in case this was deliberate to allow an existing structure to be overwritten.  Compared with the deprecated gdk_color_parse, the argument order is swapped so Gdk.RGBA.parse appears as a method in the GIR file where as Gdk.Color.parse is a function.

Adding the (out) annotation to gdk_rgba_parse causes the method to move to a namespace-level function as follows:

    <function name="rgba_parse"
              c:identifier="gdk_rgba_parse"
              moved-to="RGBA.parse"
              version="3.0">
      <doc xml:space="preserve">...</doc>
      <return-value transfer-ownership="none">
        <doc xml:space="preserve">%TRUE if the parsing succeeded</doc>
        <type name="gboolean" c:type="gboolean"/>
      </return-value>
      <parameters>
        <parameter name="rgba"
                   direction="out"
                   caller-allocates="1"
                   transfer-ownership="none">
          <doc xml:space="preserve">the #GdkRGBA to fill in</doc>
          <type name="RGBA" c:type="GdkRGBA*"/>
        </parameter>
        <parameter name="spec" transfer-ownership="none">
          <doc xml:space="preserve">the string specifying the color</doc>
          <type name="utf8" c:type="const gchar*"/>
        </parameter>
      </parameters>
    </function>

That could cause an issue if the attribute
  moved-to="RGBA.parse"
isn't respected.