Replacement of the water temp gauge with a Smith's combination oil pressure/water temp gauge is a common thing in GT6's.
The combo Smith's gauge was supplied with many MG's, and is not hard to find, especially on ebay. The bezel is sometimes different, but you can use the bezel from your water temp gauge (or better yet a busted gauge from ebay or someone's junk drawer).
Both sides of the combo water/oil gauges are mechanical gauges, not electrical.
You can/should keep your oil warning light, but to do that you'll need to put a T piece on the block so you can attach both the electrical sensor for the light & the line for the gauge.
If you don't want to scour ebay or get a new/rebuilt combo gauge from Nisonger
https://www.nisonger.com/smiths-classic-gauges.htm
Rimmer Brothers in England will sell you everything you need: adapters, gauge, etc.
https://www.rimmerbros.co.uk/rimmer/triumph/gt6/elec#Instruments & Cables
Search down the page for "combined gauge".
If you'll put 'GT6' somehow in your subject line, I'm more likely to read a post. The TR's, Spits, Stags, etc. are nice, but I don't have much experience with them & scan mostly for GT6 posts.