"I heard that they're worth more if they died during it?"
- generally yes, but many other factors contribute to value.
Most importantly, in this case, the medal is reduced in value if its "companions" are absent.
In this case, Pte WALKER would have also been issued a 1914~18 British War Medal, and possibly also a 1914 or 1914~15 Star. Given that he died in France in 1917, he may have seen the Star (if he was awarded one), but he would not have seen the other two, as they were not created until after his death. The medals, in that case, would have been forwarded to his next of kin, together with the "Dead Man's Penny" memorial plaque.
To a collector, in individual medal is a disappointment when not accompanied by its proper companions.
You might be lucky, and be able to track them down: there is at least one website that exists for that purpose.
"They were gold plated, I believe. The one I have appears to be."
- not so. they were gold-coloured, but were a "copper medal, lacquered in bronze."
Certainly I have seen many with discolourations that gold would never carry.