A complete verified list of all Pokémon in the Gamma Emerald demo is not present in the available source material. The current Early Access page says more Pokémon and a National Dex are upcoming, so a full roster should be marked待确认 rather than invented.
POKÉMON INDEX / all pokemon in gamma emerald demo
What the current page confirms
The current Early Access build covers Littleroot Town through Mauville City and lists more Pokémon and a National Dex as roadmap work. That wording indicates the available roster is still a work in progress.
The supplied gameplay sources visibly include the starter trio Treecko, Torchic, and Mudkip and discuss several encountered Pokémon, but they do not constitute a complete current roster database.
Demo roster versus Early Access
The historical demo is a separate non-canon island build and is explicitly unsupported. Pokémon shown or obtainable there should not automatically be listed as current Early Access encounters.
Use a version label on every future index entry. The current official EA page takes priority for present-tense availability, while old playthroughs are discovery evidence only.
How the index should grow
A reliable index needs the Pokémon name, version, route or condition, encounter method, and whether the entry was tested in the current build. The available sources do not provide all of those fields for every Pokémon.
Until each field is verified, keep the page at the confirmed scope level. Do not fill gaps with data from unrelated Emerald games or competing download sites.
Current Early Access facts are version-labeled. Historical Demo details, community reports, and unverified routes are kept separate and marked 待确认 where necessary.