type LongestInt = LongInt; { might get bigger than LongInt someday }
LongestInt is GPC's longest-possible signed integer type. Currently, this is the same as LongInt. On some platforms it is 64 bits wide and thus has a range of -9223372036854775808 .. 9223372036854775807.
There are lots of other integer types in GPC, see Integer Types.
LongestInt is a GNU Pascal extension.
program LongestIntDemo; var a: LongestInt; begin a := 42; WriteLn (a) end.