In '97, GTP was an 'addon' package to the GT. After that, the GTP became its own model. The SEs w/ 3800 is still an SE. It didn't have some of the options available to the GT trim.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne