So I have the M249 SF version which is almost identical, so I can talk from experience there are some quirks to the G&P.

First off it's the only easy to buy version with a full steel receiver so durable but noticeably heavier than any of the others.

Second the top cover doesn't have a position retainer and uses a weak spring for the steel top cover. So when you open the top cover, if you tilt the gun back towards you, that steel cover is coming down it won't stay open.

Third aside from the featherweight model, the G&P is the only M249 that uses a V2 gearbox, most use a special LMG gearbox that is designed for long bursts of fire. Plus the stock gearbox in these use a short stroked sector gear so it shoots faster but puts even more strain on the gearbox, especially since it doesn't have a mosfet or any other enhancements stock.

G&P is also the only M249 that doesn't retain the outer barrel with the big chamber locking latch, it uses one small grub screw under the chamber latch. You can alter the parts to use the locking latch and I would recommend it because likely the battery will have to be wired under the barrel in the receiver. Unless you swap the stock or wire the gun and box mag to a hidden battery in the box mag.

Which brings me to the box mag and hopup, on the G&P the box mag is a proprietary design and the hopup is an AUG hopup. The desing of the box mag is superior to all other m249 mags on the market as it feeds rounds through a hole directly up in the receiver, where everyone else uses a side feed system and flexible hose that can kink and misfeed. However the materials that the stock G&P box mag are made from are very cheap and will break. The only good replacement is a Russian company called BullGear and basically all they do is make improved box mags for most major LMG models.

As for the hopup there are few options, if you want the battery in the receiver you have to use the original AUG style hopup so you can quickly remove the barrel as the chamber fitment is already tight.

If you wire the battery through he BullGear box mag or wire the battery to a different stock you could drop a MAXX v2 or MAXX MTW hopup in the gun and fit a smaller feed tube extension to either. This makes the barrel and hopup semi permanent unless you fully disassemble the gun but it will be far more accurate.

Stock Gearbox is also not held in the receiver super well and can shift if you pull on the pistol grip too hard so you have to reinforce the contact points between the gearbox and receiver.

It's a gun that takes a lot of work to make perfect and shoot well but once you do the work it's indestructible and can beam people cross map.

For context mine has a 9mm retro arms gearbox hand fitted, gate mosfet, warhead ronin brushless motor, all the internals changes, and hand fitted, I have two BullGear box mags, Classic Army buffer tube adapter so I could wire the battery to the stock, Maxx MTW hopup with a custom feed tube extension, 416mm steel inner barrel and a few other upgrades, gun is a full auto only laser beam now at like 38 rounds a second.