Quite a few powers, such as power armor and gadgeteer, have a melee power (in this case, laser sword and gaunlet chainsaw respectively). What i want to know is whether it's worth it to replace them with some power from heavy weapon?

Sure, if it's fun.

It's important to note that if you're using Power Armor toggles, you can use Laser Sword, but the way the "slot" mechanics work will prevent you from activating non-Power Armor attacks.

Currently i am using a Laser Sword with particle acceleration on a gadgeteer/pet build. Would replacing it with heavy weapons work then?

No and it would lose you tons of damage.

Yes it would work fine depending on your build. Just keep in mind you'd lose the particle debuff that you may be using with Orb Cannon. But there are more than enough ways to make up for it depending on your build, stats and which HW power(s) you're interested in.

The two powers you mentioned, Gauntlet Chainsaw and Laser sword, are both tier 1, high-speed, high DPS attacks with a high energy cost. If you were to substitute something from tier 1 Heavy Weapons, you would be dropping down to a medium speed, medium DPS attack in exchange for medium energy costs and either a stun component (Beatdown), or 3 target cone with random enrage building (Cleave). Whether that's worth it or not depends entirely on your expected fighting style.

If you're looking for straight DPS and are willing to wait for a Tier 2 power, Skewer might be a really solid choice. Out of the box it is a medium speed, high DPS attack with the option to charge for some spike damage. It also has advantage options to improve either the base damage or maximum damage for either heavier DPS with tap-spam or high spike damage. But either way, it looks like you will be spending 5 advantage points to get it to full power instead of the usual 4.

Also at Tier 2, Arc of Ruin might be a pretty good choice for utility instead of straight DPS. As is, it's a melee cone capable of respectable spike damage and can be used to disorient foes. It also has an available advantage that can knock 10% off of foes physical resistance, which makes it a nice can-opener for anybody who uses physical attack (like force users, power armor users, soldiers, and any other melee heroes).

Honestly, unless you're setting up some synergies with some specific powers in other sets, it feels better to take at least two powers from heavy weapons instead of just one. Cleave is much better with a strength build and either Enrage or Howl to maintain stacks. Skewer or Annihilate can generate some impressive numbers if you can get the +100% retaliation bonus from Guard active. For that matter, charging a Skewer or Annihilate means you need some way to lock down an opponent for two seconds, which Beatdown's stun component or Devastate with Restraining Order can really help with.

But it's up to you to decide what you really want.

Currently, as I have no recharge needed, I decide to (maybe) drop laser sword and molecular self assembly for Arc of ruin w/ No quarter and Brute Strike w/ concussion. Not certain if that's good idea or not.

EDIT: Can someone give me opinion on this? I also want it to be good for an occasional PvP, which is one of the reason I want to get Heavy weapon, mainly the brute strike (cancel healing) and arc of ruin. I am not sure what's the debuff applied from Particle acceleration.

Can someone please answer, thank you.

This isn't a question you can just say "yes" or "no" to really, it depends on your priorities.

For pure damage? Probably not worth it. For style? Absolutely worth it.

I'll add that Cleave is a fantastic basic melee attack for basically anyone, since it can build enrage at a truly incredible speed - that one I could probably recommend just on stats alone.

But again, it really depends on what you're after and in what way you mean 'worth it'.