2003 Lexus GS300. Front end making TERRIBLE groaning/creaking when turning wheel, also lots of noise when any kind of side load. Ideas?

Broken U-joint or could be steering box -- get it to a reputable mechanic before it gets worse and cause an accident.

Check your struts and strut mounts & bushing. You can also get a groaning/creaking noise from sway links, tie rod end links, control arm bushings, CV axles... being that you get the noise when turning and going over bumps I'd look at struts first.

check the Lower Ball Joints.. the GS models have a known problem with them failing & if it does while your driving on the freeway & the wheel comes off.. it could be "game over" for you ... Mileage is critical - the lowest reported problem of complete failure was 68k miles.. most common failure around 80-120k any mileage over that... Just change them... there were recalls on earlier GS Models for this.. but the reality is Lexus doesn't want to pay for a recall & not enough people have died .. so their is a "technical bulletin" issued by Lexus to inspect at every oil change.. (which nobody does)