I've made a quick official version of Garry's Mod for Half-Life:Source.
This allows you to pose the models from the first game along side the models from the second game. You can also enter the maps - but there isn't any AI and some things don't work as expected.
You can download this version at the official Garry's Mod website. (You need to own Half-Life:Source to use the Half-Life:Source version of Garry's Mod)