I'm trying to simulate a transmission gate, but I'm getting a lot of warnings and errors. Can somebody help me on this?

May be inside the inverter you have another TT_MODELS component? if this is the case you might be loading the models two times. You can set attribute

only_toplevel=true,

so only the topmost instance in hierarchy will go in the netlist. If this is not the case send the

tgtest.spice

netlist produced by xschem.