When driving i can smell gas in my 2000 lexus es300

Possibly your O rings which isolates fuel injectors from engine block are getting old and letting gasoline go out. it is very dangerous since it can cause fire. do not drive.

another cause is broken loose or cracked fuel lines. this is equally dangerous since it is under hood close to hot exhaust manifold.

if smell is from exhaust manifold, which means not the smeel of gasoline but exhaust fumes, this can be caused by broken exhaust manifold or a crack on it or problematic exhaust manifold gasket.