Players will tweak their game settings and you will still get this gapping.
Different mods will do it as well - I had a map that looked fine in stock ET but was still gapped in ETPro
Convert your arches to an ase model and then it will always look correct no matter what the player does to his settings.
Here is a batch file that I use to create the ASE model - your paths and filenames will vary
REM ASE conversion batch file for q3map2
REM By MrLego
REM STEP 1: Converting raw map file to bsp file - press any key...
pause
q3map2 -meta -patchmeta -subdivisions 2 -v -game et -fs_basepath E:/Rtcw_ET/ e:\rtcw_et\etmain\maps\map2ase.map
REM STEP 2:Converting bsp file to ase model file - press any key...
Pause
q3map2 -convert -format ase -v -game et -fs_basepath E:/Rtcw_ET/ e:\rtcw_et\etmain\maps\map2ase.bsp
Pause
I have a sample map (source included) that you can download Here
EDIT: Download link updated