Garmisch - Interlaken Ost - options

Any way you do it, it looks like about 8, maybe 9 hours. On the Bahn query webpage, I can see a train connection all the way from Garmisch-Partenkirchen to Interlaken Ost, with changes at Munich Pasing, Buchloe, Zürich, and Bern taking 7h50m and, with advance purchase, costing 196€ for four adults.

Skatie, I would think option (1) or (2), your choice.
The drive from Garmisch to Lindau is scenic, following the foothills of the Alps, but slow. Same applies to doing it by train. The cost by train is €38 for 4 people using a Bayern ticket, an absolute bargain (not $38, '$' is the symbol for the dollar, they don't have dollars in Germany, there is no such thing as a $EU). You can buy a Bayern ticket at any time from any station in Bavaria. Note you should buy it from a ticket machine, it costs €2 more to buy it from a manned ticket counter. Garmisch to Lindau would involve changing trains a number of times and takes 4 to 4½ hours. You can look up times at www.bahn.de
There is a map of the train routes covered here: https://www.bahn.de/p/view/mdb/pv/deutschland_erleben/allgemein/tickets/pdfs_laender-tickets/2015/mdb_190311_streckenkarte_bay_2015.pdf
Alternatively you could had south to Innsbruck and then get a train directly to Zürich. You would then just have to buy 4 tickets on the internet, which would cost more. There is no cheap option, and the train Innsbruck to Zürich is a "RailJet", not a regional train.
Note even if you get to Lindau, you still need to go through a small section of ÖBB (Austrian railways) track before you get to the Swiss border at St. Margrethen. You would have to get a ticket for this separately.

Hi Skatie- Is your return flight from Germany? If so I would recommend keeping the car and driving from GAP to Interlaken. Especially with 4 people. Then return the car in Germany before your flight.. I did the reverse route last fall (Interlaken to GAP) and only ran into one stau on a Sunday south of GAP in Fernpass. If your return flight is not from Germany (or the country where you picked up the car) ignore this suggestion.

Have a great trip!!!

only ran into one stau on a Sunday south of GAP in Fernpass

That slow to very slow going through the Fernpass is pretty much every day. I have been through there far too many times, always get held up, always swear to go a different way the next time and always repeat myself because the alternatives are not good alternatives.

Nigel- Fernpass was worse than slow... it was turn off engine and take a short walk several times and lose count of the number of cars making a Uturn ...