I did a test with Abbasid gathering rates on a farm near my TC and this was the result (small deviations are possible due to villager position on farm, but those will be very small (1/m or so):
38/m + no ugr
40/m + wheelbarrow (+15% villager speed and +5 carry)
43/m + horticulture 1 (+15%)
47/m + golden age 1 bonus (+10%)
50/m + horticulture 2 (+15%)
55/m + house of wisdom agriculture (+15%)
59/m + horticulture 3 (+15%)
60/m + bonusses golden age 2 & 3 (2x 5% for 10% total)
60/m + house of wisdom improved processing (+8% dropoff)
considering worst case, where the added % is calculated on the base gathering rate of 38/m
in total: 103% extra (ignoring wheelbarrow) if you follow the discriptions, so that would be 77/m
considering best case, where the added % is calculated on the current gathering rate, which results in 86/m.
In any case, at least for Abbasid, the end result doesn’t seem to come close to what it should.
The last bonusses don’t seem to work at all (or capped at 60?)