Hello, I am new here and before i download any games i would like to know :
do the games have to be played off the Hard drive?
can the games be played the same way as if one purchased a hard copy?
some games are very large thats why i am asking
yes, i tried searching this topic

Define "played off the hard drive"
If you mean "can I put it on a flash drive", then that will depend on the game. But if the retail version "supported" it, the GoG version will too. And the GoG version might support it, even if the retail didn't. Consult the specific forums for each game to find out.

Problem: HDD is not big enough
Solution: buy 250GB external HDD which can fit ever single game available on gog.
Result: Lots of fun and you can even take games anywhere you want to.

If you're after something truly portable, a moderate number of games on here could work like that but would probably require a bit of work to set up. Anything that uses DosBox or SCUMMVM should be able to be configured to run everything from the game folder and not write to the registry, that way you could end up having them on a flash drive and playing them anywhere that has a USB port and XP/Vista/Win7

If you purchased a hard copy of most, if not all of the games on GOG, you would still be "playing it from the hard drive". PC games are not like console games, they are (almost) always installed to your hard drive and the only thing the installation disk is really used for after that is a copy protection check. GOG games are no different in this respect, except they are DRM-free so there is no copy protection check.
If drive space is a serious concern, after installing a GOG game, you can delete the installation package as it is not required to play the game at all. I would recommend you back up the install file to external drive or CD/DVD before you do that, just in case.

I have not tried but I would assume that almost all games on GoG would work on a external or a flash drive.
The only thing you would have to do is copy the shortcuts from your start menue to the root of the flash drive.
Note: Some newer games ( released for windows XP and onward ) might not work too well as they tend to save their savegames and configuration settings in the My Documents branch, but all dosbox games and some older windows games should work just fine.
I should mention that there is a excellent free program called Allway Sync which is free and easy to set up - It will allow you to automatically sync files to your flash and vice versa. You can set it up to work on Last Modified. This means that once you save your game, it will automatically update the save file to your flash drive, so that when you pull out the flash you have your current content. Then you say go to another computer with it where you also got allway sync set up and it will automatically copy your saves to the right directory, giving you a flawless hasslefree save system. It also has online storage to upload stuff to a ftp server or something.
Before you ask, no, I am not affiliated with them in any way :P Its just a piece of software that I have found very useful for exactly this purpose =D

Well, a hard copy pc game has to be installed on a hard drive anyway. So there is not much difference if they are downloaded and installed via the internet (i.e. GOG, Steam, Implulse, GamersGate etc...) or installed by a hard copy version.
I'm sorry if this is incredibly obvious, but your question was a bit vague... :S
Well a hard copy version of a game has to be installed to the hard drive anyway.
So there is not much difference a game is downloaded and installed to the PC's hardrive via PC game internet stores (i.e. GOG, Steam, Impulse, GamersGate etc...) or if it is installed from a hard copy version.
Sorry if this is incredibly obvious, but your question was a bit vague.

Not quite. I have 53 (I think) GOG games out of the about 175, and they take up 150 gigabytes for the compressed installers - you might need a terabyte drive to stick them ALL on, but nonetheless it's not impossible!

WOW thank you for all the answers.
HD space is not a problem. I know games have to be installed off of hard copy. Actually i didn't mean playing the games off a external device like a thumb drive.
i appologize for the vague question.
what i was hoping for was that after downloading a game i could some how convert them to the same format the hard copies were.
i haven't played a "new " PC game for some time But i remember after buying games in the past i could do a minimal install and keep the original disk in the drive.
--- OH to be clear i am taking about compact disk games not older floppy games ---
some games played music off the disk during game play - and in this case the music is stored on the HD which to me is waste of space and rather keep the music on compact disk.
so basicaly my question realy was - can the games be converted back to the disk version --then installed like a hard copy and have all the music and what ever else normaly is left on the orignal disk.
i guess ill add this since i havent played any newer PC games than civilization 3 and return to castle wolfenstein - todays games actually take up more than 4 gigs of space?

First question: no. There may be a few exceptions to that rule, but in general no. Also it isn't really common anymore on new games nowadays, to leave part on the disc rather then installing it.
Second question: yes. Not all, but a lot take up space between 1 to 7 gigs, so 4 gigs or more is rather common.

Or 17GB, like Dragon Age ;)

Hey,
to answer your question more specifically:
SOME games on GoG.com come with comparably large file that has the extension ".gog".
This file is actually a image of the CD of the game. I'm not 100% certain whether that CD is a actual retail copy of the game or one that has been modified by GoG, but regardless you should be able to make a copy of the file, rename it to .img and use your burning tool of choice to burn it down to a CD.
Then you may be able to use that CD to install the game with a minimum installation setting if the game offers such.
This is a lot of variables, so the selection of games with which you can do this might be somewhat limited. But if you already have some GoG's you might as well go ahead and try burning one of the .gog files and find out. =)

Thank you for all the help.
I guess I am going to stick with this site even if i can't play some games off of disk. I haven't found a place that realy compares.
i just hope one day they will have Blade Runner and Some of my older floppy games that now suffer from bitrot.