Any change to include the manuals of the videogames in the future?

Unlikely, since they released the first game in June.

I suspect that GoG only got the license of the games, not of the goodies (such as manuals, artworks, soundtrack, etc.)

I dont usually buy game on GOG, but do they include digital versions of the manuals?

Depends, sometimes they're available separately as extras, in other cases they're included with the game itself, only accessible from within the install directory.

In case of RE1 and RE2 there aren't any, likely due to licensing issues, as already pointed out by lockhartdcs.

If you don't mind doing a little research you could try the Internet Archive, they often have scanned/digitised versions of game manuals available for download.