The Elder Scrolls: Arena didn't found in search window
but you gain this game for free if you buy DOOM II + Final DOOM
why The Elder Scrolls: Arena don't have her own page?

P.S. Same problem with The Elder Scrolls Chapter II: Daggerfall

Both games are not sold here, but added to your account when you buy any Bethesda game.

They do not have a store page, but they have a dedicated forum:
https://www.gog.com/forum/elder_scrolls_series

That's intended, you cannot get them on their own, only with purchase of any other Bethesda game.

Greetings!!
You can download the full games for free from Bethesda's official Elder Scroll's website.

Arena:
https://elderscrolls.bethesda.net/en/arena

Daggerfall
https://elderscrolls.bethesda.net/en/daggerfall

You will probably need to tinker with dosbox a little, but as far as I remember the instructions are provided with the download.

EDIT: As others have noted you can't download these games from GOG, unless you've bought any Bethesda game.
Hope this helps! :)

Don't use those -- these! :) Like the GOG versions, all the mucking around with setting up DOSBox and applying patches has already been done; UNlike the GOG versions, there is a bunch of optional community patches and other content included (in the case of Daggerfall, anyway). (Besides, the version of Arena on Bethesda's site is the floppy disk version, whereas Wiwiland's version -- and I think GOG's -- is the CD-ROM version.)