It should be
MOI.set(m, MOI.RawParameter("TimeLimit"), 100.0);
but for this to work at the moment, you should checkout https://github.com/JuliaOpt/JuMP.jl/pull/2003, MOI master and https://github.com/JuliaOpt/Gurobi.jl/pull/216/. It should work with JuMP v0.20.
In JuMP v0.19, you can hack your way around with
Gurobi.setparam!(backend(m).optimizer.model.inner, "TimeLimit", 100.0)