How do you machamp without trading in Pokemon LeafGreen?

You can't, at least not on leafgreen... the only way to get those pokemons without trading is using pearl's GTS...

First, go into the GTS and put the Pokemon you want to evolve up for trade, but offer it for an impossible trade, like a level 9 and under arceus.

Next, do at least one trade over the GTS (it can be any Pokemon, so just trade for a dumb one)

Finally, after you have made at least one trade, take the Pokemon you offered back and it will evolve. I evolved my golem and alakazam this way... so it'll probablly work for gengars and machamps as well...

No. Has nothing to do with pearl.