That's an interesting thought, so I thought I'd take a look. Taking the Gatland era as a whole (so including the games where Rob Howley was the head coach, as everybody in the media seems to), his record against individual teams was as follows (Won-Lost-Drawn (if any)):
6N Sides (including WC warm-ups, Autumn Internationals etc)
Italy 13-0 (Home 7-0, Away 6-0)
Scotland 12-1 (Home 7-0, Away 5-1)
France 9-5 (Home 5-1, Away 3-3, Neutral Venue 1-1)
Ireland 8-8-1 (Home 4-4, Away 3-4-1, Neutral 1-0)
England 8-10 (Home 5-3, Away 3-7)
Total: 50-24-1 (Home 28-8, Away 20-15-1, Neutral 2-1)
RC
Argentina 6-1 (Home 4-1, Away 2-0)
South Africa 5-11 (Home 4-4, Away 0-4, Neutral 1-3)
Australia 3-13 (Home 2-8, Away 0-3, Neutral 1-2)
New Zealand 0-12 (Home 0-5, Away 0-6, Neutral 0-1)
Total: 14-37 (Home 10-18, Away 2-13, Neutral 2-6)
Total (Big 3 only): 8-36 (Home 6-17, Away 0-13, Neutral 2-6)
T2
Canada 2-0 (Home 1-0, Away 1-0)
Fiji 4-0-1 (Home 2-0-1, Neutral 2-0)
Georgia 2-0 (Home 1-0, Neutral 1-0)
Japan 2-1 (Home 1-0, Away 1-1)
Namibia 1-0 (Neutral 1-0)
Samoa 3-1 (Home 1-1, Away 1-0, Neutral 1-0)
Tonga 3-0 (Home 2-0, Neutral 1-0)
Uruguay 2-0 (Home 1-0, Neutral 1-0)
USA 1-0 (Away 1-0)
Total: 20-2-1 (Home 9-1-1, Away 4-1, Neutral 7-0)
Others
Barbarians 1-1 (Home 1-1)
Brumbies 1-0 (Away 1-0)
Chiefs 1-0 (Away 1-0)
Eastern Province Kings 1-0 (Away 1-0)
Total: 4-1 (Home 1-1, Away 3-0)
Gatland's Overall Total: 88-64-2 (57%)
Overall Total (Tier One only): 64-61-1 (51%)
It makes for interesting reading. He fared better against England and Ireland than I realised, worse against France than I realised (though still not bad), and as poorly against the southern hemisphere big 3 as I thought. His record against Scotland and Italy is exemplary, but they make up half of all his wins against 6N sides (25 out of 50). Together with his excellent record against Argentina, those 3 sides in blue account for nearly half of all Gatland's wins against Tier One opposition across his 12 years (31 out of 64). Throw in the fact that his overall win total is just over 50% when you take out the large number of wins (24) over non-Tier One sides and it's not exactly a glittering resume.
Couple more points of observation:
Gatland's record across three world cups was 11-7. However, if we take Tier One matches only, it's 4-7.
There were 8 summer tour series on Gatland's watch. Wales were 3-5-1 across those series. Again, if we take Tier One opposition only, Wales were 1-5. The one Tier One tour win was Argentina in 2018. The 5 tour defeats were NZ twice, SA twice and Oz, all whitewashes. The 1 draw was Japan.
Gatland's finishes in the 6N were pretty evenly spread. They were:
1st - 4
2nd - 2
3rd - 2
4th - 3
5th - 1
Hopefully Pivac reads this and it relieves the pressure.