Hi Averill
Made it to the FHC today. Interesting results but not what you are looking for. I found 2 bpts for Illeg John's between 1740 and 1750. John bpt 25 Nov 1744 s/o Bahee Curlett and John s/o Joney Cashins bpt 1 Sept 1745. neither entry listed a father. I think the John Christian who marr Cath Tear is the s/o Daniel Christian (Barry)bpt 29 Oct 1740. Certainly the math is better.

There were 2 bpts for John s/o Gilbert priv bpt 5 Jan 1752 and 9 Jan 1753. The John Christian of the 1774 will is the father of Gilbert as you surmised.
c/o Gilbert Christian and Mary Corkill
John bpt 5 Jan 1752 or 9 Jan 1753
Gilbert bpt 27 May 1755 priv bpt
Robert bpt 1 May 1757
Esther bpt 5 Feb 1759
Margaret bpt 18 April 1762
Thomas bpt 27 Dec 1764

So Gilbert is the brother of Catherine who marr John Radcliffe in the Belfast Radcliffe,s if I follow the thread correctly. IT is interesting that John left very little to Gilbert, was he doing so well he didn't need it or were they not getting along?

Catherine's parents are listed the her bpt as John and Ellin Curlett/Corlett not Joney as seen in the marriage contract for John amd Mary Corkill. Gilbert's bpt in 1721 did not list a mother.

I hope this helps sort out this family. If I can help further let me know.
Carol