Hi! To support continued development, the way I've managed it is that fractional versions (1.5, etc) are free updates, and the whole version numbers are separate products.
Gumroad doesn't support upgrades, and so when I launched 2.0, I ran an upgrade sale.
2.5 (and a soon to come 2.6) were free upgrades to 2.0, and any existing 2.0 customers on Gumroad can still get those for free.
The email to 1.0/1.5 customers was unfortunately timed because shortly after, Gumroad had a public meltdown and I pulled my products in response.
I'll probably run another sale here in the near future, so keep your eyes peeled!