I think a deeper explanation of Prisma's effect and non-specified Fusion Material Monsters is in order to clear this up.
First of all, the above people are correct when they say you cannot send 1 random GB monster to the grave for Prisma's effect; Prisma needs a specific monster to copy the name of, and if the Fusion Material Monster is unspecified (ie, the Fusion Monster saying it needs "1 Gladiator Beast monster" instead of needing "Gladiator Beast Samnite" or something), then it has no name to copy. It's similar to the reason why you can't use a Fusion Substitute to copy the unspecified Warrior-Type monster for UFOroid Fighter, or to summon Ojama Knight or that one Gemini Monster Fusion; they don't specify a specific, named monster, so the Fusion Substitute can't adequately replace it, since it doesn't know what monster it's supposed to replace.
That being said, you CAN copy the name of a specified Fusion Material Monster with Prisma, then use it as one of the non-specified Fusion Materials, as its name is now that of a specified Gladiator Beast (for example, you can use Prisma to copy Laquari's name, then summon out Bestiarii, and bring out Gyzarus, using Prisma as the unspecified GB Fusion Material, since its name is now that of a specified GB monster, and thus can fit in as the "1 Gladiator Beast").
So, to clear things up, if you use Prisma's effect, you MUST use it to copy a specifically-named Fusion Material Monster, since it's the only monster that has a name for Prisma to copy, but AFTER it copies that name, you CAN use it as one of the non-specified Fusion Material Monsters, since it now has a specified monster name that fits the requirements for being an unspecified Fusion Material.