This is just a suggestion to use ET Legacy to play ET.

It's a highly polished source port that's updated for today's systems.
https://www.etlegacy.com/

OK, but when You try to connect it says that some files are missing from original game I supose.
Any workaround for this so on ET Legacy we can play?

I'm not sure what the problem is here, All you need from the GOG version(or another install) are the "mp_bin.pk3", "pak0.pk3", "pak1.pk3", and "pak2.pk3" files that are in "etmain"(put them in the "etmain" folder for you ET legacy installation.)

Technically speaking you don't even need a separate installation as the ET Legacy Installer can download and extract all needed files.

If you didn't actually have the files though you wouldn't even be able to get to the main menu.

What you are describing sounds like one of a few problems:

1. You may be trying to join a server using PunkBuster and Pb doesn't work with ET Legacy(and I don't think many servers use it)

2. The server is using some other mod(maybe ETPro?) that is incompatible with ET Legacy.

3. You are trying to join a server with some other custom files(like 3rd party maps) and either you or the server doesn't have it set up to allow downloads. Check the "get missing files" and the "use http/ftp downloads" options in the "options/system" menu.

Not sure what OS you are on but I had this problem w/ Mint. If you are on linux you need to use the 32bit version for most servers even if you have a 64bit system... really wish this was noted somewhere on their site.