Since I haven't seen anyone else reply, I'll toss this out there in case it helps - unfortunately I can't confirm if it works or not since we don't use the CA Access Gateway (we use API Gateway).
The docs here indicate you can use HTTP Header, which would include the user-agent, to write rules on: Configure Proxy Rules Manually - CA Single Sign-On - 12.7 - CA Technologies Documentation
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<nete:cond type="header" headername="USER_AGENT">
This element indicates that a header is used, and that USER_AGENT is the header to be evaluated.
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If you can queue off that condition, then you'd just need to identify the different user agent strings you want to handle - or broad "mobile" I think should catch most - and write the appropriate action based on it (e.g., redirect to https://m.someapp/ ).