<p>Looking for a c# library that interfaces to multiple merchant gateways.
Should at minimum be able to handle PayPal and authorize.</p>

Considering that working with the gateways themselves is extremely simple, Adding support to your application for each one shouldn't be difficult.
That said, here is one that claims to have the support you want:
http://www.componentsource.com/products/ibiz-e-payment-integrator-net/index.html
Note that Payflow link and payflow pro were purchased by PayPal from verisign.