ULX doesn't have a one log option, so, disable ULX logging if you don't want day by day.
Source servers, including Gmod, I believe follow the standard Source cfg log configuration options.
Found this via Google on Facepunch actually.
// start server logging
log on
// enable or disable server logging. on is on off is off
sv_logbans 1
// Log server bans in the server logs
sv_logecho 1
// Echo log information to the console. 0 is off 1 is on
sv_logfile 1
// Log server information in the log file. 0 is off 1 is on
sv_log_onefile 0
// log everything in one file
// end server logging
Now, before you go off and do that, be warned.
The file can/will grow HUGE.
Once it starts getting large...it starts slowing down the server because the server has to read to the end of the file for every update of the log that it does.
(This is one main reason why we split up logs into daily parts...besides, we track SteamID, and I'm pretty sure IP addresses, of folks who join/leave if you choose to in XGui)