3N is explained as "4-5H; 5-S; 14-19HCP; partial stop in", ending there. The interactive BBO-SAYC (I know, not what GIB plays, but...) describes 3NT as "... 13-16... choice of 3N or 4♥". This second description sounds much better to me.
4♣ is explained as "Help suit slam try -- 3+C; 4+H; 2-11 8421HCP in C; 14-15 total points". The hand doesn't fit the description.

Help

This hand actually is NorthGIB who deals:
1♣-1♥
2♥-3N
4♣-4♥