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i stopped playing Warhammer 40k but recently got back into it my army is as follows:
2 Dreadnoughts
13 GK with halberds
4 Gk with force swords
2 GK terminators with halberds
1 GK witha a really big sword and incinerator
1 GK brother captain
1 GK librarian

how do I use them most effectively and what should i attach to them?

Flamers. Lots of FLAMERS!

Psycannons are definitely a better choice.

For the Dreadnoughts, x2 Assault cannons with Psybolt ammunition (aka Psyrifle Dreadnought). This makes a Dreadnought have two weapons that are both Str 7 Ap 4 Heavy 4 Rending, so 8 shots per walker at 24'' range.

For the GK, the halberds are fine but you will want 1 at least 1 Psycannon for every 5 GK.

Razorbacks will also be helpful.