enginesareus,
I believe
"since the self ignition quality's of diesel vs gasoline are inverse, meaning diesel is easy to ignite under compression pressure / heat, gasoline is not, octane resists that sort of combustion, so then higher pressures are needed" is incorrect. High and low octain fuels both begin combustion at roughly similar pressures and temperatures. The key factor, as BrianPeterson notes, is
ignition delay.
The characteristics of heptane (0 octane) and iso-octane (100 octane) under Homogeneous Charge Compression Ignition (HCCI) are shown below (note HCCI is not the same as the diesel or spark ignition process but often uses one or the other fuel). Diesel #2 is around PRF 62 while most gasoline is near PRF90. The figure shows similar pressures and temperatures for a given ignition point, but significantly different ignition delay.
This figure from the same paper provides more insight into what's happening during compression ignition. Note that all the reference fuels begin ignition near 810K and pass through a period of low temperature heat release then, at around 1100K, begin high temperature heat release when pressure begins a steep climb. After this point, the pressure slopes of the different fuels are similar.
From the figures, it's clear why spark ignition engines use high octane while diesel engines use low octane fuel; spark ignition engines using gasoline exploit the increased
ignition delay of high octane fuels to allow more compression before high temperature heat release while diesel engines use low octane so bulk heat release occurs rapidly upon injection of fuel into the pre-heated air.
P.S. Note time-zero associated with ignition delay in the top figure begins when piston acceleration changes sign. This trigger was selected purely for convenience and reflects behavior of the mechanical apparatus used. Per the paper, the time from change of piston acceleration sign to combustion is 0.4 ms, so ignition delays of the figures should be reduced by that time period to remove that bias.