Sorry, cannot help.
You may already know this, but I'll post anyway;
The reason there's been no response so far; no-one has extras (at least they're willing to part with).
Daval used the same back door on Gusher as a couple other machines; e.g. American Eagle, Marval; it is part number C-1-012.
Although those are common machines, back doors only come up occasionally; seems like there are more machines without back doors than back doors available.
Guys with extra back doors tend to hang onto them thinking that one day they're going to pick up a machine that needs it.
The only correct tokens for your machine will have the name "Gusher" on them; these do not come up often at all.
The Bonus or Jackpot Gusher token will be near impossible to find.
However, Gusher tokens are the same size as the many other tokens made by Daval or Groetchen and they will work in your Gusher.
These are on eBay frequently and a fair price would be $1-$1.50 each inc shipping for the common lower denominations.
Recommend you do not use quarters; not quite right size and MAY cause problems.
I picked up a NOS Gusher years ago with a full tube of 105 tokens.
Apparently the machine was supplied with the following distribution of Free Play tokens:
3FP-61
5-25
10-10
15-5
25-3
JP-1

- Gusher JP token