Hi. I was wondering if there is a way to install Antix with the gnome desktop from the outset, or do you have to install Antix"as is", then install gnome later.

You have to install antiX as is and then gnome. If you want to have an antiX-gnome, I suggest installing antiX-base or antiX-core and then install gnome.

Will antiX-gnome run as smoothly as antiX-icewm? or am I dreaming of a fairytale land?

I had antiX with Xfce and it ran pretty smoothly.
But I don't think it would run any better than Debian with the same kernel. Debian usually runs quite fast GNOME or whatever desktop environment.
It obviously takes more time to load the desktop environment than IceWM and of course Gedit takes more time to open than Leafpad (but probably not more than Geany) so it depends on the applications.
If you install GNOME over full antiX then you'll still have those lighter alternatives like PCManFM and Leafpad side-by-side with Nautilus and Gedit.
GNOME is quite light for what it offers, is your machine very old? Only if it's very old it would make much difference.