No time player means that the session won't have save/load option and reset, basically.
yeah you just don’t have the scratch construct, can’t scratch, that’s pretty much it
other people other than time players can time travel however, if they have the appropriate timey items, Gamzee does use Aradia’s music boxes at one point I think
however they are not going to be very intuitive at using them for more than very simple things, like how a fish just doesn’t have a good way to do complex tasks like write or climb a tree
Dave himself mentions that John’s Retcon super breath power, despite not being itself a time power, was messing with time in ways that John just doesn’t intuitively understand the way a Time player would