The gearbox fluid looks like it has contaminants on inspection. Fluid can also look discoloured and very different to fresh gearbox fluid.
There is a light similar to the one pictured here showing up on the dashboard. Different manufacturers sometimes have slightly different warning light symbols but it should be similar to the one pictured.
When changing gear the gearbox makes a loud grinding noise. This will be a more common occurrence on cars with manual gearboxes but can also happen with automatics. There can be various reasons for it but it always presents as a loud, unpleasant sound of mechanical scraping/grinding.
Changes between gears have become difficult, the gearstick no longer moves freely between gears, it sometimes seems to get stuck or won't go into a gear properly.