I have started downloading witcher 2. It had completed around 3 gb of download but when I started downloading again later in the day it started from around 1 gb. Last night again after having kept it for download for hours it had reached 5gb but later when i started again it was 4gb. How to solve this issue? this happens every time when i start the download after I have exited the downloader.

GOG Galaxy 1.1.27 beta

I would imagine it's dumping incomplete files. So when you download you're getting 1 or two complete install files and a partial file, which is deleted when you stop the download. Later it picks up at the beginning of that partial file again. You're not really losing progress. So sorry that your internet can't download the whole thing at once, but I'm guessing that that's how it's working. I may be wrong.

Best of luck.

Use https://www.gog.com/downloader if you want an installer download instead of a full unpacked game. It's much preferable to using Galaxy in its current state, and will be even in its 5.0 Beta state.

Did you know, you can download it straight from your account page, without having to use Galaxy.
And GOG has split it up into several chunks, so you can just download however many chunks you want at the same time, so you don't lose anything. ( :

They will generally be split in the max size a Fat32 system could handle, which is typically 3.9Gb. Although they might have dropped it to 2Gb so you could better fit them on DVD's when burning your own copies, i forget exactly what they go for.

MD5 hashes:
1ffa67383248c9ec37ef6fb42f05f8f7 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25.bin
9e722929981ec4f6a6bbfc66751ffcb7 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-1.bin
80f3542c5606222d2f6c5cf3c8de3f54 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-2.bin
c472a5d12d3dbea108f2893b8506c193 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-3.bin
7f205072da57863541a3df7a16c90d80 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-4.bin
aa129c4f2806712b66505a1d29b2e5ee *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-5.bin
85e71e98baa71ed02274c9dbc0093712 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-6.bin
bc5837b2e46474168e87718f0a6131b8 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-7.bin
f3482bc507821570e0df363706a370de *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-8.bin
4926fd89d16ed4ec89c6ede74fbfd34a *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-9.bin
62fe0a17713891aaa98b32e5be86674a *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-10.bin
f909926bae4b436e27ac71276deeeeaf *the_witcher_2/setup_the_witcher2_ee_3.4.0.25-11.bin
d1be88e5fdc33b930011fcc42e4d8897 *the_witcher_2/setup_the_witcher2_ee_3.4.0.25.exe

Galaxy is slightly flawed and I would stay a long way from it.Follow the other's advice.

Galaxy is really bad at reporting what it does. Assuming you haven't manually deleted the .download and .download.state files from the temp folder, it will continue downloading from where it was, though it will state downloading while it's verifying the already existing files. Your download progress shouldn't be lost, and you should be able to verify it by checking the speed with which it progresses when you first start the client.

Yeah i do see .download and .dowload.state files in the folder, tho when i start the dowload the rate at which it progresses is same as if it's dowloading, so i don't think it's verifying those files, even when i check resource monitor it shows that gog using my entire net speed which means tht it's downloading the lost progress and not verifying it... I think it's dumping the partial files as said above

I think I'll continue with the dowloader , if things get worse i'll switch to downloading from browser, thanks a lot for everyone's help

It really shouldn't be doing that though. Best I can suggest then (other than what you are already doing of course) would be to file a ticket, following the instructions here.
Still, good luck with the downloading :)

Even with Galaxy I prefer downloading the backup setup files rather than install it directly.
firstly, I get a standalone installer. And then, the download size is much less I think.

And sometimes I have experienced this problem. And though I am not entirely sure (never paid much attention) I have probably seen such downloads later getting a boost in terms of downloaded file size. I may be wrong though. :P
EDIT: Someone correct me if I'm wrong.