Inner tubes?

https://planetx.co.uk/products/panaracer-inner-tube

Something like this (the 700c 35-50mm option) - there are plenty of similar inner tubes.

Even if you're tubeless then a spare inner tube is still essential.

Personally I'd go tubeless on that width tyre (although if it's not tubeless ready don't bother). If sticking with tubed then TPU tubes are the 'new' thing, mostly as they're very light and have very good rolling resistance. Apparently they're not really anymore susceptible to punctures than a butyl tube (but do need a specific type of patch to repair), also avoid ones with all plastic valves and minimal reinforcement where the valve meets the tube as they're more prone to failure. That said I only have one as a spare (as I ride tubeless) so others can comment better on their experiences. As a spare though they're great, half the weight and volume (at least) of a butyl tube.

I guess latex is the other option but I'm not sure I see the point now that TPU tubes exist, I did try latex years ago but got fed up of pumping up tyres all the the time (and I don't think they are repairable but not 100% on that).

Any innertube with a range that includes 35-37mm will do. Err on the larger side (i.e. one lableed 35-47 not 21-35).  

I use TPU tubes, they have upsides and downsides.

Pros:

The material isn't elastic so when punctured the hole remains small and they go down slowly. 

They probably don't puncture any more commonly than normal tubes, if the object has made it through 1-2mm of rubber, Kevlar, polyester and whatever else then it''ll go through the tube.

Light, and pack down incredibly small as a spare.

Cheap if bought from ali-express. A £4 Ridenow tube is worth it for the weight saved compared to a £6 generic butyl tube in a shop. 

Cons:

The material isn't elastic so you can't re use them in another tyre.  They stretch to that tyre size.

I had a bad winter for punctures. Might be luck, might be the tubes.

The patches are a bit rubbish.  They adhere poorly and forget about it in even in damp foggy conditions let alone rain.

I'm not 100% convinced about the rolling resistance. In a light tyre they feel heavy compared to latex or superlight butyl tubes. If I had the time and could be bothered I'd measure how much two identical setups bounced because I suspect the TPU tube damps it a bit.  Doesn't seem so noticeable in heavier gravel setups though. 

Expensive if bought in a shop. No way is it worth £30 Vs a £10 60-70g butyl tube.

For a gravel bike I'd go tubeless. It's more faff in the garage, but less faff on the trail. But in the short term stick with butyl tubes, better to be out riding again and fix the odd puncture than kill your rekindled love for cycling with a marathon faff session trying to get tubeless or TPU tubes to work 😂

Nobody has mentioned it above, but a lot of rims are quite deep these days and often cheaper tubes don't have the valve length to stick through enough. Those Planet X ones should be OK at 48mm unless your rims are properly aero. 

I think I'd struggle to patch a tpu tube on the go, so I tend to carry a butyl tube as a spare in case I'm having a really bad day. If I'm carrying multiple spares, some might be tpu.

Tube Rolling resistance tests;

https://www.bicyclerollingresistance.com/specials/tpu-inner-tubes

Summary, in those test TPU come out slightly better than butyl.  1-2 watts per tyre so even with an average FTP you're under 1% difference on a bike worth of tubes.  That could be 2 minutes on a 100km ride.

Vittoria do latex tubes that officially cover that sort of width tyre now, the 30-38mm variant with 48mm valves, I've sworn by the 25-28mm versions for my road bike in recent years. £10.99 at

https://www.tredz.co.uk/.Vittoria-Competition-Latex-Inner-Tube_128983.htm

Before you apply anything like the 7% British Cycling membership discount.

Tube Rolling resistance tests;

yea..... but they 'feel' different.  Maybe it's like 29ers or radial tyres, feels slow, is actually faster.

If the big benefit of tubeless for you is the self sealing puncture protection, you can get that by putting sealant in an inner tube, and none of the messy setup or hassle when changing tyres.

none of the messy setup or hassle when changing tyres.

It is of you've had a puncture and all sealant has glued the tube to the inside of the tyre and rim. Makes it all an absolute **** to remove. Actually seems to make a significant to how the tyre rolls too. 

Tubeless on gravel bikes is definitely not in question, right? We're talking about what kind of *spare* tube to carry?