You know, this is a good point. What can they actually do? Lock you out of the 5th edition GSL? Sue you for breech of contract? It seems like the loss of use of the GSL is the only thing they can feasibly do, which means that having it exist after the termination of the license doesn't make sense. Of course, the intent is to have you think you cannot, even if you can.
Under clause 6 of the GSL, a publisher makes a contractual promise:
(i) not to manufacture or publish any product pursuant to the OGL that features the same or similar title, product line trademark, or contents as a GSL product (per 6.1, 6.2);
(ii) if they publish a product under the GSL that features the same or similar title, product line trademark, or contents of an OGL product, to cease manufacturing and publication of the all other OGL Products which are part of the same product line (as reasonably determined by WoTC) as that OGL product (per 6.1).
In addition, a publisher under the GSL agrees that:
(iii) its breach of these contractual promises will cause WoTC irreparable damage which cannot be readily remedied in damages in an action at law, and may additionally constitute an infringement of WoTC's IP rights, thereby entitling Wizards to equitable remedies, costs and reasonable attorneys’ fees (per 10.4);
(iv) its breach of these contractual promises may cause irreparable injury to WoTC for which WoTC will not have an adequate remedy at law, and that WoTC will therefore be entitled to apply to a court for extraordinary relief, including temporary restraining orders, preliminary injunctions, permanent injunctions, or decrees of specific performance, without necessity of posting bond or security (per 11.4);
(v) each and every remedy will be cumulative and in addition to every other remedy provided hereunder or available at law or in equity (per 11.4);
(vi) it is responsible for all legal costs, including WoTC's attorneys’ fees, associated with any action required to enforce the GSL;
(vii) it recognizes WoTC's rights and interests in its IP (including the content of the core rulebooks), and that those rights belong exclusively to WoTC (per 10.1).
On the face of the contract, therefore, a breach of the clause 6 provisions could result in WoTC getting an injunction to stop the publisher publishing those breaching products, could result in an account of the publisher's profits (conceived of either as "gains-based" damages, or as "equitable remedies" mentioned in (iii) above) and could also result in ordinary damages for any loss suffered by WoTC as a result of the breach.
In an earlier GSL thread Clark Peterson and I discussed some of the practicalities of this. He took the view that injunctive relief would be hard to get, and as he is a practicing US lawyer I'm happy to defer to his judgement. I think it would be hard for WoTC to prove loss - a necessary prerequisite to ordinary contractual damages - and Clark also thought they would be hesitant to open their accounts to scrutiny (which they would have to do, were they to try and prove loss). He is probably right about that.
Clark nevertheless thought (and obviously still thinks) that clause 6 is pretty serious. My own feeling is that the IP issues are the most likely problems a defaulting publisher would face (and this is what Clark seems to be most worried about). To see why, consider the following scenario:
A publisher signs up to the GSL, and thereby contractually agrees that WoTC owns all the content in the core rulebooks;
A publisher then publishes a GSL product, thereby agreeing not to use the OGL in respect of the content of that product;
A publisher then wants to publish some or all of the content of that GSL product under the OGL, in breach of clause 6.
If any of the content in question appears in the 4e core rulebooks then, notwithstanding that it is also found in the OGL-santioned d20 SRD, I think the publisher in question may have real trouble arguing that they have WoTC's permission to publish that content.
The publisher could point to clause 10.1, under which they have agreed that:
they will not utilize any WoTC IP (other than GSL-licensed IP) unless they have entered into a separate licensing agreement with Wizards authorizing such use.
This clause indirectly recognises the existence of the OGL. The problem facing that argument, however, would be that the publisher under clause 6 has promised not to use the OGL for the content in question.
The resultant possible finding of non-licenced use of WoTC IP, combined with the contractual acknowledgment by the party of very broad WoTC IP rights, don't look good for the defaulting publisher.