Some update on this topic: I tried to use the gdsl file created by ggarcia24, and also checked a fork. Although it is possible to add support for every plugin, I could not find a way to support some parts of the basic pipeline structure.
For example gdsl alone is not enough for the IDE to properly identify global variables and shared libraries. I suppose the whole jenkins library has to be linked inside the project somehow. Also the agent any expression will always be rejected by the IDE since it interprets the any word as a method, not a parameter. Finally I could not find out how to handle the typed parameters like enums (eg. changeRequest comparator) or time units (java.util.concurrent.TimeUnit). Not to mention some tiny details like order and max number of sections/directives.
Based on these, I think it is impossible to create a gdsl for full IDE awareness of the declarative pipeline. It can improve the UX for some extent, but will never help with the most trivial syntactic issues. However a different approach could solve this problem. The best would be to change the declarative syntax for more strict logic (like yaml), and investigate the new linting possibilities. The other way is to improve the REST validation endpoint in Jenkins to report with more useful messages.