Sue them! 32Red sued 32Vegas for having a similar name that created confusion among customers.
Since you had yours first in 1997, it is Gold VIP Club that has infringed your trademark to the extent that although different, the name is close enough to mislead potential customers into thinking the two casinos are the same. It was evidence that this confusion took place that 32Red used in order to win their case over their rights to have sole ownership of "32" for online casino names.
You can argue that the use of BOTH "Club" and "Gold" in a casino name creates a similar, if not greater, level of confusion, and win the sole right to use both words in a casino name.
Gold VIP Club probably chose their name to create confusion among customers trying to avoid the rogues, and their spam clearly intends to add to the confusion by using different variations on the name.
Unfortunately, by not donating much resource to it, it is Gold VIP Club that seems to have come along first, giving the impression that you chose the name after Gold VIP Club came along, and thus dropped yourselves right in it.
Gold VIP Club will be pleased that recipients of their spam might think they are joining a Redbet brand, rather than the notorious rogue operation of Gold VIP Club.