Each player client has to have his own Forge + Galacticraft. Your server website or info must tell clients which versions they need, you can maybe give a link to our mod downloads page to help people.
There is no way for a server to "push" the Galacticraft mod (or any other mod) to clients.
And obviously the clients need Galacticraft installed on their side, so that they have the rocket models, the blocks, the entities and all other visual features - these things do not exist in vanilla!