OK, so when I look up 'gestr' in Zoëga's dictionary it shows:

gestr (-s, -ir), m. (1) guest; (2) a royal retainer of inferior rank.

Now, refering to the chart provided at the end I would decline it like
this:

Singular
Nom. gestr
Gen. gests
Dat. gesti
Acc. gest

Plural
Nom. gestir
Gen. gesta
Dat. gestum
Acc. gesti

But, as the experienced of you problably notice, the Singular Dative
is incorrect and should be simply 'gest'

(according to http://hem.passagen.se/peter9/gram/g_sub_i.html )

Now, in this frustrated state of mine, I see no real use for Zoëga's
dictionary if these things are not mentioned. Even with llama_nom's
(thank you very much llama_nom) "conversion chart" it is possible to
possibly recognise this word as 2nd Declension but not the absense of
the -i in the Sing. Dat.

I agree it would be easier to learn by way of memorising stems, but
HOW? I do not know of a dictionary that gives a word's stem. I ask
you, how do you learn your vobulary or how do you make use of Zoëga's
dictionary?

Thank you very much,
Sebastian