Is there any secret to unlocking the Halo 2 Battle Rifle REQ for use in Warzone? Or do I just need to keep spamming Gold REQ packs in an attempt to unlock it?
There is no secret to unlocking. Each of the REQ packs contain a set of slots which spawn items from a given pool of options. The slot and pool control the weighting for randomization, with each potential option being controlled as part of the definition for that pack.
The certification is a durable, meaning that you can have at most one. Pool options that contain certifications often contain a set of durables. The weight of each durable that you have in your inventory is set such that the pack will not produce another of that item. Since each slot in a pack is guaranteed to spawn an item from the pool, the more durables from a given pool that you have inventory, the greater the chance you have to get the other items from that pool.
All of that is basically a long-winded way of saying that the more packs you buy, the more durables you'll be granted. The more durables you have, the greater the chance that you'll get one that you do not have, such as the H2 Battle Rifle.
So, your best bet is to keep buying packs.