You'll be limited to circa 25mm tyres by the rim brakes.Here you go
Here you go
Nice looking bike👍
You may not get much larger volume gravel tyres on that bike.
Post a pic of the gap between the underside of your brakes and the top of your current tyres.
You may also need to consider the amount of space near the bottom bracket.
Are you using rim brakes?
Apart from frame clearance, you also need to consider if the tyres will slip through the brake calipers easily. You may have to deflate the tyres each time you remove a wheel. Even if you deflate and open up the brake caliper, you might find it difficult getting through.
I found it impossible with a 30mm snow tyre, but on this, the rubber was not very flexible.