UKH

I've been on canal towpaths quite a lot and am realising gravel tires would be better. My fork is as follows: CUBE CSL Race Carbon, 1 1/8"- 1 1/2". The current tires are Continental grand prix four seasons. They're 25mm. Was just wondering if I could fit some slightly wider gravel tires in and if it would even be worth it. Maybe just get a cheap mountain bike at some point like a Specialized Rockhopper. Thanks

From a brief look it looks like the 28mm might be the widest fit. 

Not really gravel bike territory... 

Even if you got new forks, the rear may be limiting as well. 

N+1 time

Right yeah, thanks, would be a normal road bike with some slightly more knobbly tires. These might improve things and still be good on the road. Tempted to try them:

https://winstanleysbikes.co.uk/panaracer-gravel-king-semi-slick-700c-tubele...
 

Like Dave said, you'll be limited by tyre clearance - especially on the chain stays, which is generally the narrowest point.

I'd love to put something more knobbly on my hybrid, but it's limited to 28mm slicks for road.

I'm generally a fan of Panaracer, but not sure you or I would be gaining much with those ones.

Have a look at schwalbe cx tyres for something knobblier. I put 30mm ones on my road bike for off-road riding. Try and measure the clearance on your frame and brake calipers, if it has rim brakes.

I've also found it has max tyre clearance of 28, so unfortunately no you won't be able to turn it in to a gravel bike, nor even a CX bike. Sell and buy a more suitable one