Since lowering is a gerund here, why is it that "It’s probably more a lowering of some [financial] positions into the summer", not "It’s probably more of a lowering of some positions into the summer" or "It’s probably a more lowering of some positions into the summer" or "It’s probably more than a lowering of some positions into the summer"?
All four of these constructions have distinct meanings.
- a more lowering of - this is ungrammatical. It means that the positions had already been lowered previously and have now been lowered again. The correct phrase replaces more with further.
- more than a lowering - this means that what happened was something bigger, more important, or more significant than a (mere) lowering. It's very likely, though not guaranteed, that lowering was part of what happened, but it definitely wasn't all of it.
- more of a lowering - this is used to state that the previous description was incorrect, and the item at hand is more like a lowering than whatever was said before. The two things are on a continuum, and the action should be placed closer to lowering.
- more a lowering - this is very close to more of a lowering, but there's much less equivocation. It's appropriate to call it a lowering, and not whatever was said before. The hedging from more is usually done for the sake of politeness; saying "well, I think it's really this" as opposed to outright stating that something is wrong.
You can parse Galliker's remarks this way:
"Some people have said the reasons for this are [something]. I'm not going to say that they're wrong, but I think the real cause is shifting off risk by reducing positions."
He's expressing it politely, but the bulk of his message is that he significantly disagrees with whatever the other analysts said.
As for the second one, does gauge metaphorically refer to a certain index for the European insurance industry here? Is lower here an adjective, acting as an object complement?
There's no metaphor here. Gauge is used as in Collins definition 7: an assessment, test, or measurement. In this specific case the article is financially technical, so you can read gauge as rating, outlook, assessment or guidance. Gauge is definitely not an index or set of companies; it's referring to a single prediction about the collective performance of a specific industrial sector. It's not clear from the excerpt who issued the assessment, but presumably it's more or less common knowledge to those who find it relevant.
Lower is acting as an adverb modifying led. The use of led tells us that British insurers saw the greatest reduction of expectations, but most if not all European insurers and the sector as a whole had their predicted profits/revenues/etc lowered. Led is used in the sense of leading a group; in this case, the group is European insurers and they are being led towards worse performance.
You can read it like this:
British insurers had the largest reduction in predictions of financial performance among European insurance companies, who all saw some lowering of expectations, after a report showed U.K. prices for new-car insurance fell for a record 10th quarter.