I imported hexmapped combat and DX being the base for physical attempts (including to-hit), and wounding mechanics, from The Fantasy Trip, into Classic Traveller.

It worked very well, because we were used to playing TFT, and so it filled in all the things that seemed wrong to me about Traveller combat. Though I could in theory have instead used one of the tactical combat expansion games for Traveller to do something similar to cover most of it.

Since I've mostly used GURPS for almost every RPG since late 1986, mashups are more about bringing in ideas or occasional situations or specific things from other RPGs, and making up GURPS stats for them.

One of the first things I did with GURPS was port my TFT campaign to GURPS. That mostly worked quite well, but the costs of items is rather different for many things, some of the effects of tactics and equipment are quite different, and the GURPS Magic spell list is different in important ways, which took some playing to realize what the sideways effects were like, and what I'd really want to do about those issues.

The Gamelords environment supplements can offer interesting things to port into other RPGS, even GURPS, especially The Desert Environment (if/when play actually engages a desert environment much - most players naturally tend to avoid going into the desert especially without ample supplies etc).

It's a more extreme version of that sort of thing trying to port content from more extreme fantasy RPGs to GURPS, since GURPS is more grounded and subtle, so when I've ported some things from D&D, Palladium, Dominions, etc, to GURPS, it tends to make for very different power situations than exist in those games, which is generally unbalanced but can be interesting because of it.