I'm using 4x metal posts. These will be painted galvanized and "exposed". Rails and pickets will be inset to the 4x.

I have one side that is street facing, and i want the post abutting a brick garage wall.

The question is, should I put post as close to brick as possible? I can get it flush, but should there be a small air gap? If yes would you fill the gap or trim some how? Even if I get 100% flush there will still be gaps at grout joints.

I'm not sure what you mean by 4X post. 4" square posts?
Are your pickets horizontal or vertical?
Also - will you be hanging a gate off that post?

Typically you want concrete all the way around the post to secure it, so don't install RIGHT at the house.

I'll be using 4x4 10ga tubing. There will be vertical pickets between the 4x4. There will be a gate, but farther down the line.

Conditions prevent me from getting 360 access to set, and I'm OK with that, there will be other posts I can fully set.

Don't put the post against the brick. Leave it about 4" away and cantilever the last little piece to the house. Make sure the picket doesn't touch the brick to make it a little harder for termites to get in.

I think I'll pour a footer and have a base plate welded to my post, then anchor the post to footer. That way I can get as close as I need, and have ability to remove if the paint fails in future .

Any flaws with this logic?

I would leave the same width gap as the fence boards have
Assuming your concerns about
paint rubbing off
moisture consequences w/dissimilar materials affecting one/other
wind forces creating rubbing/maintenance
etc.

You are not saying whether to attach post to brick veneer, so ... good

Excellent primer = less painting later of course

More work than I was looking for, but I'm glad I went the base plate route! Thanks for input.