For lower gears, you can go the easy way or the significantly less easy way. The easy way is to get a freewheel with a 28 tooth largest cog. You might also need to get a new, longer chain, but all in all, pretty easy (assuming the current freewheel comes off with no problems). Downside: a 42x28 gear is not all that low (which would have been heretical statement 50 years ago) and no indexing is possible.

The significantly less easy way involves swapping out a bunch of parts and likely cold-setting (bending) the rear triangle. This would involve swapping out the crankset for something that accommodates a significantly smaller chainwheel (like down in the 20s or at least the low 30s - personally, I'd go with a triple chainwheel configuration up front but others prefer a compact double)), installing a new bottom bracket that is compatible with the new crankset, a new rear derailleur to handle the chain wrap required for the new gears, and possibly a new front derailleur (maybe, maybe not). If you want indexing, you will likely have to go to 7 or 8+ speeds, which would mean all of the above, plus spreading the rear triangle to accommodate (to 126mm for 7sp, 130 for 8sp+), plus a new rear wheel (certainly a new hub, probably with a freehub/cassette instead of a freewheel), plus new shifters (downtube, barcon or brifter, your call, and depending how many cogs are on the back wheel). I'm probably forgetting something, which someone will no doubt point out.

All doable, and there are lots of resources (including here and in the Mechanics BF forum) to help you through, but it would take some doing. Fortunately, that Griffon is more than a good enough frame to justify pretty much any improvements you want to make.