I think maybe you're looking at the offer upside down - perhaps?...

I think maybe you're looking at the offer upside down - perhaps?
The offer is that for every GMM share, they will be recompensed with 1.65 GXY shares.

It is GMM that has to keep evaluating what they are sitting at - if not exactly right now, then by the time the merger occurs. GXY holders can keep looking over and seeing if GMM is holding decent headroom for a profitable holding to be made in the short-term.
Its going to get very interesting there....

but as far as ATO considers scrip changes from mergers - and you may want to double and triple check this based on your own specific case - when you acquire shares as a scrip deal, there is no CGT event at that time, and the resulting shares (in Company B) are considered to be bought when you first acquired them (in Company A)

- ie the date you bought GMM is the date those resulting GXY shares are assumed to be purchased on.