Disclaimer: I couldn't find anything regarding on Dmax/Gmax Pokemon's HP changes and immunity to phazing, so in this post I'm assuming that both still remain with the mechanic changes.
Gonna echo what I said on Discord. Regular Dynamax and Terastal will remain the same in most cases, the only difference being that both mechanics are easier to check and less game deciding, more in Dynamax's case. Dynamax requiring an item and removing set-up is also a huge blow to offensive Pokemon. I will say that Terastal got off pretty easy. Sure, the power drops are significant but not super impactful as giving up an item for a mechanic that lost its flexibility. Terastal will still be strong since changing type match-ups is what gives the mechanic so much of its strength.
G-Max on the other hand is really interesting. They work like Megas in a way and open up some unique strategies. The only drawback is, again, the inability to hold an item. However, unlike D-Max, you can select a dedicated Pokemon and instantly G-Max it, again like a Mega. Changing abilities and keeping 3/4 of the original movepool is honestly what makes this mechanic so much better than regular D-Max, although not all of them are created equally.
Here are the Pokemon that are the best imo. I'm gonna talk about some dexited Pokemon, assuming that they will be added to the mod due to their G-Max formes. I will also use all moves that they have access to in NatDex, idk if anything of this is applicable.
- Leech Seed effect on a strong attacking move is great and with Flower Veil making it immune to Burns is pretty great against Scald. Just sad that it needs to be a special attacker to function with its G-Max move. Swords Dance can still work with Power Whip, EQ and Knock.
- Cannonade giving this thing Leftovers is nice and with Bulletproof it is a good answer to Gholdengo and it can generally holds in own in that match-up.
- Huge Power replaces and with Volt Crash being stronger and having no recoil, Pikachu has a niche imo. At the end of the day, it's still Pikachu but it can tank a hit, hits stronger with Volt Crash and having great moves in Knock and Espeed is useful. Probably a meme pick, but it has potential.
- Another meme pick, but having Anger Shell and actually having decent stats unlike Klawf, this Pokemon can do some damage. Foam Burst also gets boosted by Sheer Force which is phenominal. It's still rather slow with 75 base speed, so even +1 isn't fast but I think against a weakened or unprepared team, this will do pretty well.
- Thank God, this does not have Aurora Veil effects. However, strong Freeze-Dry and Ice Scales is pretty nasty. It does not have recovery and it is weak to Stealth Rock which will hold it back, but it can solidly check Pokemon despite that and is a nightmare for Water-types.
- Ngl, this is the best imo. Snorlax always had longevity issues and Replenish can actually replace a "useless" item with a good one. Gaining a form of recovery after using your strong STAB is great and combine this with Curse, Rest and Sap Sipper, which annoys Amoonguss, Ogerpon and Rillaboom. With the Dmax bulk, I think that this Pokemon will outclass Blissey in 90% of all teams.
- This Pokemon gets annoyed by Steels pretty hard, but having a Pokemon that spreads Poison so easily is phenominal. Knock Off and Spikes are also great support options for it. This one's really niche but has potential.
- Drum Solo is stupid strong and with Grass Pelt, this Pokemon can set-up on many Pokemon. It cannot abuse Grassy Glide as well as it wants to due to it loosing the ability, but it can still work late game, or Rillaboom can focus on SD breaking with its strong new STAB. Although, regular Rilla is better most of the time.
- The issue that this Pokemon has is its Stealth Rock weakness, but I think the Gmax forme has more value than Rillas Gmax forme. Protean is great and all, but it cannot fully abuse it anymore. Fireball is great for an SD set since it can hit Unaware Pokemon like Clodsire, Clefable or Tera Dozo and Dirge, while Heatran does not want to switch into it either.
- Gholdengo. The reason why you'd want to use this Pokemon. Okay the added bulk and Iron Barbs is also great. Missing Leftovers is pretty nasty for it, but gaining the ability to remove hazards in a desperate situation is nice and you get extra bulk.
- Gaining an immunity to EQ is extremely good against Pokemon like Clodsire, Dragonite, Gliscor and Ting-Lu. Volcalith also making the target weaker to Fire is also great against those Grounds (ik Dnite isn't Ground shut up), or against neutral Pokemon.
Unlike Grimmsnarl, which really prefers Light Clay, Hatterene makes better use of its G-Max. Gaining more special bulk with Light Screen is nice if it wants to use it for its own purposes but Light Screen and status absorption is great team support. It can use the effects of Magic Bounce until the late game, until a point where it can set-up a Light Screen and make one of its teammates sweep.
I don't want to touch on the other Pokemon because they either suck, smth like Appletun or Sandaconda, their G-Max doesn't improve them in a meaningful way, like Charizard or Gengar, or their base forme is better with the freedom of an item, Urshifus and Melmetal for example.
I'm excited for the mod and its meta, this stuff really looks interesting.