Olivia here from the Shopify team. Thank you so much for sharing your question with our network.
At this time, it is not possible to disable a payment gateway pending a certain order value. The way payment integrations are built require them to all be present at checkout, or disabled altogether.
Is there a particular reason you would like to limit transactions under the dollar value to a certain gateway? There definitely are options to limit total order amounts, either minimum or maximum, through the use of a third-party app like Order Limits if this could meet your needs?
Take a look and let me know what you think.