The 2015 gxsr 155 has a rear drum brake. So as you say the rear wheel has been off, I suspect that the drum brake has been adjusted badly OR the adjustable cammed nut that is used for adjusting the brake is in the wrong position. This could also be why the clutch has started making a noise etc. It's perhaps failed due to the rear brake binding, as thid will putting a heavy load on the clutch leading to slippage, overheating & failure. Especially as you described the bike as 'feeling as though it was pulling a heavy load.'
If the clutch has failed, replacement can be done quite cheaply but get that diagnosed/confirmed by a trained bike mechanic first. As it would be a good idea just to make sure the engine didn't have any other associated issues.
As far as the rear drum brakes go.. the adjuster is a recessed nut/cam on a shaft directly from the foot brake. This cammed nut should sit located properly in an obvious specific 'U' cupped position. If its not in this possition or its adjusted too tight your rear brake will bind, just as you describe.
If your not sure on doing the following, either check your manual or download it and read the section about rear brake adjustment, its pretty easy. IF YOUR TOTALLY UNSURE THOUGH, TAKE THE BIKE TO A GOOD MECHANIC!
All the following items are adjusted together and affect each other due to suspension movement... So you'll need to make sure that the chain is adjusted properly, the rear wheel is aligned correctly by checking the axle alignment against the rear swingarm axle markings. And then that the rear brake is adjusted properly.
It's easier if the bike is either on a trackstand, it's mainstand (if it has one) or carefully jacked up so that the rear wheel is just off the ground.
MAKE SURE THE BIKE CAN'T SLIP OR FALL SIDEWAYS!
Don't totally undo.. But just loosen off both the rear wheel main axle nuts & washers, and also the two 10-12mm axle adjuster nuts at the very rear of the swingrm, so that the wheel can slide a little in its sliders/runners.
Now undo the rear brake cam nut a good few turns so that it wont affect any axle/wheel adjustments.
Now that everything is loose. Give the rear wheel a few firm knocks from behind to move it forward slightly in the swingarm runners. You should notice the chain loosen.
Now you can start adjusting everything back up.
Whilst CONTINUALLY checking the chain tension, slowly turn the smaller rearmost swingarm adjustment nuts clockwise on both sides..
NOTE: Whilst keeping your eyes on the above also watch the main axle nuts and washers positions in relation to the markings on the sides of the swingarm, these should line up EQUALLY on both sides, meaning that the rear wheel is located correctly in line within the swingarm!!
CHECK CHAIN TENSION AGAIN and AGAIN!
If further adjustment is necessary. Repeat the above untill you get everything correctly to spec.
NOW RETIGHTEN THE MAIN AXLE NUTS TO CORRECT TORQUE!!! You can also just nip-up very slightly the smaller adjustment nuts, only enough so that they won't come loose. DO NOT TIGHTEN THEM RIGHT UP!!
Now you can adjust the footbrake.. Just turn the cammed nut untill it starts to touch the pin in the brake lever rod. PRESS THE FOOTBRAKE to test.
If the brake pedal feels too loose, tighten the cammed nut little by little. KEEPING AN EYE TO MAKE SURE THE RECESS IN THE NUT IS LOCATED/SEATED CORRECTLY.
Allow some movement in the footbrake before the brake actually bites, otherwise it will bind everytime the rear suspension moves upwards.
As mentioned though.. If you're at all unsure here, have a qualified mechanic do this job for you.