I can't say there is a standar package for an analog IP. But usualy, there is always something missing...
Any way, it depends what software they used in design. If you whant to convert in your software, that might cost extra. Basic, an IP must have, as you said, gdsII file for layout, netlist, but is better to have the design schematic, models for the devices, necessary libraries, rul files for DRC, LVS, as many as possible documentation, specs, meassurements, tests, test conditions, and the list can grow.
The conclusion is that for the money you pay, you ges as much as possible.
Read this article: **broken link removed**
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stefano2m