Looks like I have a problem with Unreal Tournament 99: when I try to play the fan-made mods like "Gothic Resurrection" or "Xidia Gold" through the oldskool amp'd mod, this mod just cannot read them! It even doesn't shows the name of the campaign ("Xidia Gold") in the menus (where should be the name of Map pack), just "Custom maps" and the list of missions of Xidia below in the menus "New Singleplayer game".

Is it typical? How to manage this? I have a GOG installation of UT GOTY.

Last I checked, Oldskool Amp'd needs files from the original Unreal. Try grabbing those from Unreal Gold and putting them in their corresponding folders in UT99 (do NOT overwrite).

Some mappacks such as ONP, Xidia/7Bullets and Zephon can now be played on the latest version of the first Unreal, but I'm not sure what mods were necessary in order to play them (it may have been XCoop or something similar).

Not really much reason to use oldskool anymore given that we have 227i (and 227j is getting cleaned up for release at the moment).