Best place to define plugins' constants?

[mvlcek]

The user can then define the constant in gsconfig.php, but if he doesn't, the default value is defined in the plugin itself.

That was what I meant: only for customisation, as default values would be set (your way) by the plugin -- just like GS does with all those #-commented defines.

So you don't think it's a bad idea using gsconfig.php for these things..

No, I think it is a good idea to use gsconfig.php for plugin specific constants, too, as long as the plugin does not force the user to enter them. This way all non-default settings are in one file. The default values for the constants defined in the plugin itself should be selected to be the most reasonable ones for the majority of users.