Hi all,
After a 1-20 campaign in pf2, our group has gone back to 5th edition, changing the GM.
Our GM is open on using some OD&D rules.

A few other players are porting versions of their characters to this new world. One player is making a clone/simulacrum of one of their characters and I thought about bringing my own character as a retired adventurer that has grown old and must now pick up the pace. Ring rust and old age have brought their prowess back to that of a 1st level character.

The character was a Gunslinger that liked to dual wield crossbows (Pistolero subclass).

One additional piece of information: during the last campaign we destroyed the moon (its tears used to power up magic items, etc)

I was thinking of building a gish-like version of that character, exploring the idea that they now use magic to have their bodies do what they used to be able to do. My concept would be that the old character now has one of the moon's tears and is able to channel its powers.

Our party is a fighter (samurai), a monk (shadow) and a cleric that has yet to choose their domain.

I was considering Warlock with the new True Strike cantrip and a crossbow. Thought about going Fathomless, but just couldn't come up with a concept as to why my offworlder gunslinger would venture at the sea. While Pact of the Talisman could be a good way to translate what I'm aiming for, it does not synergize well with the crossbow. 

The main idea was 'i've got this cool piece of astral magic that now lets me do stuff'

As for my playstyle, I tend to like playing Defenders, Strikers and Controllers.I'd like to be able to meaningfully contribute in combat, wouldn't like to lag too far behind the monk and samurai in dpr (do not have to surpass them)

TLDR/In Short. I used to play a gunslinger, and want to play an agened version of them that uses magic to be able to be a crackshot.

For moon-theming use Celestial Warlock, take Pact of the Blade + Thirsting Blade, grab the XbowXpert feat and combo your crossbow strikes with Spirit Shroud.

Alternatively, you could play a dex-based paladin :
1st level Start a Variant Human with Crossbow Expert feat and using a handcrossbow, combine it with the Divine Favor spell for extra damage.
2nd level Ask your DM if he'll let you take Archery as your Fighting Style, if not take Protection.
3rd level Choose if your DM will allow you to us the OneD&D Devotion Paladin take that, otherwise take Vengeance.
4th level take the SharpShooter feat, and now cast Bless on yourself at the start of each combat.
...
...

You might also want to pick up the Fey Touched feat for Hex + Misty Step at some point.

There is a Gunslinger that was previosly ported by Matt Mercer for one of the Vox Machina characters. You have to enable Critical Roll content, and I can't find a sourcebook for it anymore and the drop down game rules do list it, but not sure if it's freely available to everyone (Under the Fighter as a subclass). Sorry, I bought too much content over too many years...

Quote from Grayfax >>

There is a Gunslinger that was previosly ported by Matt Mercer for one of the Vox Machina characters. You have to enable Critical Roll content, and I can't find a sourcebook for it anymore and the drop down game rules do list it, but not sure if it's freely available to everyone (Under the Fighter as a subclass). Sorry, I bought too much content over too many years...

 
It is a free fighter subclass available to everyone, and it’s not part of any sourcebook so that’s why you can’t find it in one.