Isn't the easier way to just make it say:
At the beginning of your end step, put an arms counter on Arming Gala. Creatures you control, in your hand, in your library, and in your graveyard get +1/+1 for each arms counter on Arming Gala.
Or even:
At the beginning of your end step, put an arms counter on Arming Gala. Creatures you control get +1/+1 for each arms counter on Arming Gala.
The reason I worded it the way I did is because Arming Gala specifically does not buff creature tokens until they hit the battlefield. Your wording would give token strategies running this version a massive boost.
I think I would just let it create an Emblem/a Conspiracy token that could have counters on it. Obviously Wizards doesn't want to do this but it would work. And if Conspiracies can't have counters, it could just have a number on it instead and let all creatures you control get +/+ after that number.
In this way it would function exactly as the digitally printed card from Alchemy.
Could also use experience counters. Wizards also wouldn't do this but it could be done.
The upside of an emblem is that the buff stays when the enchantment gets destroyed. The downsides are that I can't think of an elegant wording that wouldn't also buff token armies in an unintended way (basically the same problem MilesOfficial's suggestion runs into) and the fact that a second copy of Arming Gala (or a recast off of
Eternal Witness) gives you a second emblem which doubles the buff. You also don't want to use experience counters, because there are other cards that generate those, which means this suddenly gets buffed by a number of cards in an unintended way.
The closest approximation I can think of using the emblem tech is this, using a little bit of free-form Magicese:
When Arming Gala enters the battlefield, if you don't already have one, you get an emblem with "Nontoken creatures you control get +1/+1 for each arms counter you have."
At the beginning of your end step, you get an arms counter, then put a +1/+1 counter on each token creature you control.