Do any gap pads dry up?

TOP256GN has no heatsink interface or exposed pad, heat dissipation mostly takes place through pins rather than plastic case.

Thanks yes i appreciate what you say...but our FSGM0565 has obseleted and we need to shoehorn something else in the same old assembly...the easient thing, is a TOP256GN, which due to its high profile body, means its case is nearer the metal enclosure, and so we can gap_pad its plastic case to that enclosure...i appreciate heat transfer wont be great out of the plastic body....but its better than nothing, and if it passes the thermal test, then we dont care anyway.

Thing is, will the gap_pad dry out?

I actually want to get rid of topswitch altogether, but everyone is topswitch mad, wherever you go...i'd rather just use a ceramic to220 fet and current mode controller, and screw it to the heatsink like they used to with the FSGM0565.

"Gap pad" could refer to an enormous range of materials and vendors, can you be more specific?

Many thermal pads are based on silicone, and those can contain some residual oils which may "sweat" or "leak" out due to severe thermal cycling. To be clear, this is not normally observed, and seems to vary wildly between different materials. I do not know if such "sweating" is actually correlated with a change in thermal performance though. The main concern over the sweating is contamination of the rest of the assembly, or the general environment (matters a lot for certain applications).

There are many non-silicone-based thermal pads which advertise that they do not leak or outgas. But whether this means their thermal performance is more stable over time, I don't know.

Also as noted above, a thermal pad between just the chip and the chassis/case likely won't do much, as most heat will still escape through the pins/pads to the PCB. You will likely get far better results by filling the gap between the entire PCB and case, even if it's on the opposite side of the chip. In such a case you may consider using a dispensible/liquid compounds, or just potting it.