Tiny Neck gap

[markaroni]

There is a small gap between the ledge on the neck and the body (I’d say about 1-1.5mm). Is this something I should worry about before gluing on the neck? Is sanding the heel to get rid of the gap necessary?

[McCreed]

Is this something I should worry about before gluing on the neck? Is sanding the heel to get rid of the gap necessary?

The short answers are, no and no.

As long as the bottom of the heel (tenon) is fully seated in the bottom of the pocket and the neck angle is sufficient to get the lowest possible string action, it's fine. (BTW, most of the time the neck angle is best adjusted via shimming rather than sanding the heel)
That gap under the fretboard is normal even on Gibsons. It does not get glued to the top of the body or need to contact it. The sides and bottom of the tenon are the contact areas for gluing.

[Simon Barden]

I would normally just fill it and paint to a matching colour (masking around it as necessary).