". . . Am I allowed to give a gift to a client after they put me down as the referral? . . ."
You can't go through life being afraid of your own shadow !
Most Real Estate Rules and Regulations are really quite practical, and not all that bizarre . . . . or causing you to deviate all that much from normal human courtesy or making gestures of thanks.
If prohibiting token gifts of appreciation were the case here in Vermont, I would be living in the Cross Bar Hotel for the rest of my life for giving subscriptions to the local Newspapers. Who's going to prosecute me for that ?
I doubt that Texas is much different. You probably heard about illegal Commission "kickbacks" that should be disclosed under RESPA. Check with your own Broker and consider it as Continuing Education.