I found ProCreate the best to use for press moulds, but in your case I'd go with GS. If you want to make a two-part mould, I'd personally look at other options.

Surely this is a very expensive way to go? Why did you find it best? (what properties etc?)

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AFK with real life, still checking PMs

Price is similar/equivalent to GS (assuming you buy the GS that comes in two parts and not the ribbon).

I like it more because because it has no memory like GS does, and because you have much more control over the working time.

You don't need to wait for it to cure a bit before working with it, and you have quite a long working time once it is mixed. You can still slow the curing time down in the freezer (and continue the following day), or speed it up by putting the model+putty in the bottom of a mug and filling it with boiling water (by the time the water has gone cold, the putty is cured).

Mixing the resin and hardener in different ratios has more notable properties than with GS (again, IMO). For moulds, I use a lot of the grey in the mix (say 65:35) and for the casts, I use more white (60:40). The flexibility of the mould is important to get the best out of complex shapes (I cast Necromunda figures' heads in a one-piece mould with a single side split for example), and the firmness of the cast is important as it makes it easier to trim any flash off and file rough areas smooth.

I also find the colour (grey) easier to see than the dark green of the GS.

2 part moulds, I saw something on coolminiornot I am sure. Will have to take another look. MAybe need the hardness of illiput.

You can also mix GS & Milliput/Magic Sculpt. It tends to be easier to work with & holds detail better than GS solo.

Frankly, I've never been a fan of using just GS.

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WAAARGH!!
The Lost & the Dipped