If you'd provide full system specs we'd know what we're dealing with. Including where exactly the M.2 slots are and where you're planning to put what drive. Include chassis brand model too. The chassis you have, number and kind of fans you have, their position in the case and other components in there will help us get a picture of the general air flow in the case.
Although you can't predict how hot anything will run
at idle and
under load. Only when you have everything in there running you can monitor temps at idle and under load and try to remedy anything if necessary.
So get the system going. If under 'normal' heavy load the SSD temps don't go beyond 70C you're good. If it does momentarily spike to +80C and goes down quickly to around what is OK you don't need to worry. Again, if it stays above 50C during idle you can get a heatsink.
there is No side panel on my case. its always open
Generally, this doesn't help air flow and in many scenarios/situations makes it worse, as in components run hotter. Although this depends on case model. A low quality badly-designd case with no proper intake and output fans might benefit from an open side panel.
idle is around 50-60 , but i just bought this nvme to install my OS on.
That's not too bad. As I said something between 40-50C is OK. If you can keep it 40ish it's even better.
is there a separate heatsink to purchase?
Yes there are different kinds. The better ones usually come with their own thermal pads prestuck to the sink. User would only have to peel off some thin protection plastic/film (covering the thermal pads) before sticking the sink on the SSD.
how about using thermal pad UNDER it , so it make contact to mobo?
(its really easy thing to do because this nvme have only 1 IC and the rest of PCB board have Nothing else in)
No you should'd do that. The motherboard is not a heatsink, it's PCB as you said. The sinks that come with some drives by defualt or the ones you add to them go on top of the SSD; not under it.
the thing that concerned me is that, its almost behind my GPU
Where is behind? When a normal tower case is upright the Graphics card is perpendicular to the motherboard (parallel to the 'floor'. In that position the M.2 slots are either above the PCIE slot/graphics card or under it.
As said above, post a picture of the innards of the chassis and we'll see what's what.