If you ever want to tame one, bring a bola. Think of the gallimimius as a parasaur in maximum overdrive. It runs away at lightning speed, meaning you either need a) a quick acting tranq with low dps, or b) a bola and either a club or tranq arrows(bring a primitive tool though, they are very fragile creatures for their size.
Also, bring some narcotics. The torpor drains fast if left unsupervised, and that starts the whole process all over again, part 3 goes into more depth.
One more thing, bring a tough mount/tough gear. They spawn in the redwoods, which are at the center of the island(for context for new players, rexes, direbears, purlovias, and carnos oh my), which you also have to cross the swamps(titanoboas, sarcos, baryonx, megaphirana, and kapros). If you can’t pick it up, you actually have a two part mission 1) finding and taming and 2) the escort mission. Bringing some gear that can get you through both segments is key.