Adobe After Effects is a film software. The whole compositing system is totally different from the particle system of acknex.
Actually there is a particle editor in After Effects, but its quality is...not very good and you can't compare it to acknex.
For particle effects in gs use Easy Particle 3.1 (
http://www.easyparticle.com/).
It exports in c-script but you can change the codes to lite-c easily.