Since a grayscale file converted to RGB is going to have equal R, G, and B values, the gamut of the color space doesn't really matter. But the gamma curve of the color space will to some degree; if the source and destination spaces don't have the same gamma, then the equivalent of a curve adjustment will be done on the image data to convert from the source gamma to the destination gamma. But if they are the same, then the grayscales can simply be duplicated to make the R, G, and B channel values. If your grayscale file is gamma 2.2, use Adobe RGB, but if your grayscale file is gamma 1.8, use ProPhoto.
As to which grayscale gamma is best, 1.8 is closer to the tone response curve of printers, so gamma 1.8 images need less munging to be converted to the printer color space than gamma 2.2. Gamma 2.2 is closer to the native response of monitors, so I recommend using Gray Gamma 1.8 and ProPhoto as editing spaces and default color spaces for print files, and Gray Gamma 2.2 and sRGB for web and monitor-display-only files, especially singe most of these files are JPEGs and 8-bit-only.