These functions (which are part of StatsBase.jl, not Julia itself) do not support the dims keyword. There is an issue for adding this functionality.
You can use geomean.(eachcol(x)) instead, but it will return a vector instead of a row matrix:
julia> using StatsBase
julia> x = [1 2 3 ; 4 5 6]
2×3 Matrix{Int64}:
1 2 3
4 5 6
julia> geomean.(eachcol(x))
3-element Vector{Float64}:
2.0
3.162277660168379
4.242640687119285