You have a beautiful gold coin, Corina, and it is valuable like all gold coins. Don't try to clean it or polish it. That always ruins value. Figuring the value of your coin is a little trick. Look at our 5 steps below.
First, the only date in this series which commands strong collector value is the coin from 1926. This information comes from the coin catalogs. Only 50,000 gold 20 francs were issued in 1926, and that's not many. In comparison, there were more than 3,000,000 gold 20 francs minted in 1930. A 1926 Swiss 20 francs catalogs for $500 in fully uncirculated condition, $250 if circulated and well preserved.
Second, the primary source of value for your coin is its gold content. The catalogs also report that your coin contains 0.1867 troy ounces of gold. Therefore, the gold in your coin is worth 0.1867 times $910 US dollars, the current spot price of gold. This equals $170. (The price of gold changes every day. Look it up on web sites such as kitco.com. The word 'troy' is understood when dealing with ounces of gold or silver.)
Third, your coin sounds to be fully uncirculated and protected as such in a holder. This fact adds some value to your coin, say $50, bringing the value of your coin to $220. If your coin were not fully uncirculated, the added value would probably be $10 or so.
Fourth, if you might sell your coin to a dealer, he or she would probably (but not always) charge a fee of about $30 or more, bringing the value back to $190.
Fifth, if you might sell your coin to a collector, he or she would probably (but not always) be willing to pay a premium for your coin, again about $30 or more, making your sale price $250.
To keep things more confusing, consider the two pictures of Swiss 20 fancs shown to each side here. Do you see how one of them has oodles of eye appeal and the other is, well, lackluster? Guess which one has more value. Of course it is the one with better eye appeal. Collectors pay high premiums for coins with good eye appeal.
Further, you will be amazed, Corina, at the wide variation in coin pricing. You can find this coin on the Internet for sale at prices ranging from $200 to $1100 -- all the same coin and all in roughly the same condition. The advertisements for the $1100 coins have all sorts of hype, like 'extremely rare' and 'brilliant uncirculated.' Don't be fooled.