[morzh]

If the coil is the one that goes to the grid, the gauge is less important and can be a bit smaller even. It can be larger if you could fit it.

If the coil is the anode one (the load of the tube's output) go with at least the gauge that was used. While resistance is not awfully important, it i somewhat important, dependent on the current the tube is working at. A bit larger gauge will not hurt you either.

Note: if it is an IF filter-transformer, then you should try to stick to the same gauge as the length of the winding determines the inductance (it is a part of the formula) and deviating too much from it might shift the alignment too much, at least in theory. Simple RF transformer that is not tuned is less critical to it.