score:7

var x = gigs.where(g=>g.acts.select(a=>a.id).contains(7));

these two queries also return the same:

var x = gigs.where(g=>g.acts.count(a=>a.id == 7) > 0);

var x = gigs.where(g=>g.acts.firstordefault(a=>a.id == 7) != null);

score:14

essentially the same as mike_g, only more verbose syntax and using equality.

var mycollection = from gig in qry
                   where gig.acts.any(act => act.id == 7)
                   select gig;

just an edit to bring comments to the answer:

actually query is for an id on a member (artist) on the act object that can be null.

new query:

var mycollection = from gig in qry
                   where gig.acts.any(act => (null != act.artist) && (act.artist.id == 7))
                   select gig;

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