wrong location? (n00b question)

[Dexter Filmore <Dexter Filmore gmx de>]

So I installed gtk on my Slack 8 recently for I thought "whatever was installed, maybe get the latest version". 
Then I discovered that 1.2.10 is already enclosured in Slack 8 and friend told me it's not too good having 2 packages of it simultaneously installed. So I kicked the one I installed myself out, deleted the package from Slack and reinstalled it.
I'm still kinda linux noob, so one may forgive me that.

Funny thing is: on a tty "echo $PATH" displays full path with /opt/gnome in it, on X as user as well, but in a terminal in X as root via su command the /opt/gnome path does not apper. How come?

Next thing: is this the right location anyway? Tried to compile gtoaster recently, ./configure said "hey u dont have gtk" and other apps, too - they all look for gtk and devel stuff in /usr/local/somewhere as it seems.

If someone could please shed some light on this? I'm close to insanity spending days on the simplest things.

Dexter Filmore