Hmmm. Here's more information that adds to the issue without clarifying much:
GURPS Banestorm (p. 25) introduces three spells.
- Detect Banestorm "confirms or denies any current Banestorm activity within the specified range."
- Seek Banestorm "tells the caster the direction and approximate distance of the nearest banestorm."
- Identify Newcomer, "when cast on a person or object, tells the caster whether a banestorm brought it to Yrth within the specified period."
All three are in the Gate College, although other Gate spells are "rare and unreliable" on Yrth-1. Many of these others (though not Seek Gate) are subject to a -25 penalty (!) due to the effects of the Banestorm (p. 26).
Detect Banestorm has Magery 1 as a prerequisite. Seek Banestorm has Detect Banestorm; Identify Newcomer has Seek Banestorm. In contrast, Seek Gate requires Magery 2, Seek Magic, and one spell each from 10 other colleges.
I infer from this that big, hairy, unnatural phenomena like banestorms are relatively easy to detect with magic; gates are subtle and require correspondingly more finesse. Nexus portals are (mostly) natural phenomena and about as flashy as gates, so probably do fall in the same level of difficulty.