It can, as of the recent releases.
Avi, do you use Pharo as your main development environment?
Re:Smalltalk is stable and growing
by spooneybarger on Wednesday December 16, 2009 @03:24PM
(#999638)
gnu smalltalk can run seaside but it tends to lag behind pharo as pharo is the target platform ( gemstone, cincom et al also lag behind to some degree ).
What I'd be more curious about is whether GNU Smalltalk can run Seaside...